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scott wrote:
> I've definitely got annoyed by some sites where you hit the back button
> and it takes you to a site you were looking at 10 minutes ago, because
> all the things you've been doing on this site haven't actually loaded a
> new page...
Yeah, this is a lot of fun.
The other one is "JavaScript links". You know, where the "link" isn't
actually a hyperlink, it executes some JavaScript, which then tells the
browser to change URL. And because it's scripted, you can't Ctrl+click
to open the URL in a new window. (Or if you do, the existing window
changes URL anyway, so you *still* don't have the original page. At
least you can usually use the browser history to go back a page though.)
And then there's the links which pop up a "window" over the current
page. You know the kind - the whole page goes grey and a funky little
box appears over the top of it, and you can't do anything with the rest
of the page until you dismiss the box. (Modal dialogs suck for a reason!)
Lots of shopping sites like to take you to a completely different page
as soon as you've bought something, which also tends to thwart any
attempt to used tabbed browsing to nagivate around. You've got a page of
items, but as soon as you buy one, the page goes away. Great. Now I have
to somehow navigate back to where I just was...
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Invisible wrote:
> Seems reasonably clear...
Hmm, I wonder if I could use this to implement distributed MVC, like in
my Final Year Project, but without requiring a custom client.
Of course, as far as I can tell, I need a real web server to even
experiment with this stuff, so...
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scott wrote:
> The tutorial was good up until step 7.
Hmm, maybe I should write my own?
> 2 + 2
That was easy, eh?
> 2 + 3 * 4
The usual operator precedence rules apply.
> (2 + 3) * 4
You can override operator precedence manually in the usual way.
> sqrt 144
This is a function call. (Notice the lack of brackets!)
> div 10 3
This is a 2-argument function. (Again, notice the lack of any brackets.)
It performs integer division (i.e., division to the nearest whole number.)
> 10 / 3
Does the same calculation, but with floating-point arithmetic.
> 10 `div` 3
This does the same thing as the first one, but it looks more natural.
The backticks work for any 2-argument function.
> sin 1 + cos 1
This is sin(1) + cos(2).
> sin (1 + cos 1)
This does a different calculation.
> sin -1
This gives an error, because it thinks you mean "sin - 1", which is
invalid. (Yes, we all think it's very annoying.)
> sin (-1)
This works properly.
> "This is a string."
> 'X'
This is a single character.
> "X"
This is a string that only contains 1 character.
> ""
This is an empty string.
> [8, 3, 11, 2]
This is a list of numbers.
> ['X', 'Y', 'Z']
This is a list of characters. Note that a string *is* a list of characters!
> ['X', 3]
A list can contain elements of any type, but all elements must be of
*the same* type. This example is therefore invalid. (Nice clear error
message, eh?)
> sort [8, 3, 11, 2]
> reverse [8, 3, 11, 2]
> sum [8, 3, 11, 2]
> 3 `elem` [8, 3, 11, 2]
> 4 `elem` [8, 3, 11, 2]
Haskell has many, many list functions.
> [1, 2, 3] ++ [4, 5, 6]
This joins two lists together.
> [1..10]
This is a shorthand for writing lists.
> sum [1..100]
Add up these numbers.
> product [1..100]
Multiply these numbers. (Notice that even though the answer is huge,
Haskell computes it exactly.)
> take 3 [5,4,6,3,7,2,8,1,9]
Get the first 3 elements.
> drop 3 [5,4,6,3,7,2,8,1,9]
Remove the first 3 elements.
> [5,4,6,3,7,2,8,1,9] !! 5
Get the 6th element. (The first element is index zero, not index one!)
> filter odd [5,4,6,3,7,2,8,1,9]
Filter a list according to a condition.
> filter (> 5) [5,4,6,3,7,2,8,1,9]
Filter with a different condition.
> map (*2) [5,4,6,3,7,2,8,1,9]
Double all the numbers in the list.
> map toUpper "This is a string."
Since strings are just lists that contain characters, all list functions
work on strings.
> sin
You can't print this out, so you get an error.
> :t sin
This tells the interpretter to only print the type signature, not
*execute* the code. (This is an interpretter command, not a Haskell
command.)
> (5, 'x')
This is a "tuple". Tuples can contain any number of elements, and they
can be of different types. Their main use is in returning several
results from a single function call.
> partition odd [5,4,6,3,7,2,8,1,9]
This splits the list into two lists, containing odd and not-odd numbers.
The lists are returned as a size-2 tuple.
> :t fst
Take a look at this type signature. Can you figure out what it means?
> fst (5, 'x')
Did that do what you expected?
> let x = 5 in x + 1
This is how you define local variables. (The interpretter unfortunately
doesn't let you define things and then use them later. Everything has to
go into a single giant let-expression.)
> let x = 5; y = 8 in x + y
This is how you define multiple variables.
> iterate (*2) 1
This starts with 1, and applies the "times two" function to it over and
over again, creating an infinite list of results. (Again, the
interpretter doesn't display the whole thing.)
> let factorial n = product [1..n] in factorial 6
This is how you define a new function. (Normally it wouldn't be a local
variable, but the interpretter unfortunately doesn't allow top-level
definitions.)
> :t (+1)
This is a function that adds one to a number.
> :t (*2)
This is a function that multiplies a number by two.
> :t (+1) . (*2)
The "." operator takes two functions and combines them together. Here
we've made a function that takes a number, multiplies it by two and then
adds one. (Note carefully the order - RIGHT TO LEFT!)
> map ((+1) . (*2)) [1..10]
Apply this function to a list of numbers.
> :t div
This is a 2-argument function that does integer division.
> :t div 10
This is not a syntax error. It's a 1-argument function. WTF?
> :t div 10 5
This is a calculation that produces a number.
> div 10 5
This divides 10 by 5.
> map (div 10) [1..10]
See if you can work out what's going on.
OK, I'm going to stop now...
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Invisible wrote:
> Of course, as far as I can tell, I need a real web server to even
> experiment with this stuff, so...
You have a real web server.
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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scott wrote:
> Can the script intercept this and provide its own "back" functionality?
GIYFT!
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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>> Of course, as far as I can tell, I need a real web server to even
>> experiment with this stuff, so...
>
> You have a real web server.
I do, but it can only serve static HTML. (Plus it's quite a lot of work
continually logging into the web interface to re-upload your files every
time you edit 1 line of code.)
I think to try this out properly I'd have to set up a local Apache VM or
something. (This requires me figuring out how the hell to set up
Apache.) Alternatively, I could just write a trivial Haskell webserver
that will log what requests the browser is sending in response to by code...
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> Hmm, maybe I should write my own?
It's very useful for an absolute beginner, these are the things you need to
know that a lot of tutorials assume you somehow magically know already.
> > :t fst
> Take a look at this type signature. Can you figure out what it means?
>
> > fst (5, 'x')
> Did that do what you expected?
Well it seems to return the first item from a tuple.
Is it just a shorthand for "take 1"?
Ah no, it seems those list operators don't work with tuples :-(
> > let x = 5 in x + 1
> This is how you define local variables. (The interpretter unfortunately
> doesn't let you define things and then use them later. Everything has to
> go into a single giant let-expression.)
So this means "let x = 5 in (the following expression)..."
Now Step 7 in the original tutorial makes sense :-)
> OK, I'm going to stop now...
OK I need to download the compiler if I am to actually learn any of this...
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Invisible wrote:
> I do, but it can only serve static HTML.
Uh, no. Every major OS comes with a real web server.
Now, if what you mean is "I don't have my own computer", then that's a
different question.
> (Plus it's quite a lot of work
> continually logging into the web interface to re-upload your files every
> time you edit 1 line of code.)
Unless you're using an IDE, yes. ;-)
> I think to try this out properly I'd have to set up a local Apache VM or
> something. (This requires me figuring out how the hell to set up
> Apache.)
Depoending on your OS and distribution, this can be pretty trivial.
But if you're actually going to get into AJAX programming, you're going to
have to learn more than you can by asking people here to explain it to you. ;-)
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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scott wrote:
>> Hmm, maybe I should write my own?
>
> It's very useful for an absolute beginner, these are the things you need
> to know that a lot of tutorials assume you somehow magically know already.
The hard thing about writing a tutorial is that by the time you're
experienced enough to be an authority on the language, half the stuff
that trips newbies up is so utterly "obvious" to you that you forget to
even mention it. (Plus you're all excited and you want to jabber
incoherently about the cool stuff, not the dry old "this is how you
escape a newline character".)
>> > :t fst
>> Take a look at this type signature. Can you figure out what it means?
>>
>> > fst (5, 'x')
>> Did that do what you expected?
>
> Well it seems to return the first item from a tuple.
>
> Is it just a shorthand for "take 1"?
>
> Ah no, it seems those list operators don't work with tuples :-(
It's all in the type signature.
[Int] means a list of integers. It can contain any possible number of
integers (including zero or infinity). [Char] means a list of characters.
Tuples, on the other hand, have a different type for each size, and each
combination of element types. For example, (5, 'x') is of type (Int,
Char), but (5, 'x', False) is of type (Int, Char, Bool). As far as the
type system is concerned, these two types are completely unrelated.
Because of this, you cannot write a function that works on any possible
size of tuple. It's impossible. (You can write a *method* that works for
any size of tuple that you've manually written a definition for, but
that's about it.)
In truth, if you're using tuples bigger than about three or four
elements, you really ought to define a custom type for this stuff
instead of relying on tuples. Tuples are really for simple, quick things
like returning multiple results from a function, or for key/value pairs.
Stuff like that.
...and this whole explanation is a good example of the kind of thing
that's hard to explain in an interactive tutorial. ;-)
> So this means "let x = 5 in (the following expression)..."
Indeed yes.
> Now Step 7 in the original tutorial makes sense :-)
Yeah. It's simple, it's just explained badly...
>> OK, I'm going to stop now...
>
> OK I need to download the compiler if I am to actually learn any of this...
I can talk some more if you want?
But yes, this (alpha) web thingy lacks several important features:
- You can't define things and refer to them later. Each expression is
processed independently.
- You can't write multi-line definitions.
- You can't do I/O. (And the error message if you try is cryptic to say
the least!)
- You can't use all of the standard Haskell libraries, just the handful
the interpretter offers.
In a real Haskell interpretter, you can usually do stuff like
> let factorial n = product [1..n]
> let choose n k = (factorial n) / (factorial k * factorial (n - k))
> choose 3 5
In the online interpretter, you'd have to do
> let factorial n = product [1..n]; choose n k = (factorial n) /
(factorial k * factorial (n - k)) in choose 3 5
This, coupled with the horrifyingly buggy command history, is just no
fun at all.
Are you actually interested enough to want to play with it more?
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Darren New wrote:
> Invisible wrote:
>> I do, but it can only serve static HTML.
>
> Uh, no. Every major OS comes with a real web server.
Really?
> But if you're actually going to get into AJAX programming, you're going
> to have to learn more than you can by asking people here to explain it
> to you. ;-)
No, I'd just like to see if I can do a small proof-of-concept thing and
make it work. ;-) Let's face it, DOM is way too hard in the first place...
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Invisible wrote:
> No, I'd just like to see if I can do a small proof-of-concept thing and
> make it work. ;-) Let's face it, DOM is way too hard in the first place...
Then take static files and request them via xmlhttprequest. That's how I do it.
Make up files text1.txt text2.txt text3.txt and so on, then write a loop in
javascript to sleep a few seconds, xmlhttprequest the next file in the list,
and when it shows up take the data and populate a DOM element with it. It works.
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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Invisible wrote:
> But yes, this (alpha) web thingy lacks several important features:
>
> - You can't define things and refer to them later. Each expression is
> processed independently.
>
> - You can't write multi-line definitions.
>
> - You can't do I/O. (And the error message if you try is cryptic to say
> the least!)
>
> - You can't use all of the standard Haskell libraries, just the handful
> the interpretter offers.
They seriously should steal features from http://tryruby.org/
- You can define variables and refer to them later. Example, type a=1, press
Enter, then type a+a.
- You can write multi-line definitions. Try this one:
5.times {
print "Hello"
}
When it parses the first line, it will notice the statement is incomplete
and let you continue it.
- You can do I/O (fake filesystem). The whole chapter 4 of the tutorial is
about I/O and file handling.
- You can do HTML and see the results via a custom library made for the
tutorial. Try:
require "popup"
Popup.make {
h1 "Hello world"
p "foo"
}
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Nicolas Alvarez wrote:
> They seriously should steal features from http://tryruby.org/
Well, the page even *mentions* Try Ruby.
I guess the guy just hasn't written the code yet... A few days ago, ":t"
didn't work. And now it does. (Annoyingly, GHCi uses ":t" as an
abbreviation for ":type", but the website doesn't accept ":type", only
":t".)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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> The hard thing about writing a tutorial is that by the time you're
> experienced enough to be an authority on the language, half the stuff that
> trips newbies up is so utterly "obvious" to you that you forget to even
> mention it.
Exactly.
> In truth, if you're using tuples bigger than about three or four elements,
> you really ought to define a custom type for this stuff instead of relying
> on tuples.
OK. I take it a custom type is a bit like a struct in C-type languages, in
that it can hold any number of different types?
> I can talk some more if you want?
Once you know a few basic things I think it's best to move on to simple
examples (small multi-line samples, not those artificial ones needed for the
web).
> > let factorial n = product [1..n]
> > let choose n k = (factorial n) / (factorial k * factorial (n - k))
> > choose 3 5
Exactly. Even I can work out from that one how functions are defined now
:-)
BTW what would happen if I typed "choose 3 (-5)" instead?
I'm just trying to think of a subject for a good example ... how about a
card game? Firstly how would you represent the cards (tuple or custom type
for the suit/value?). Then I assume you would just create a list of "Cards"
to represent a group of cards that someone was holding (say 5 cards). How
would you then check to see if any two cards in that list were the same
value, or all the same suit, etc?
> Are you actually interested enough to want to play with it more?
I was interested enough in J to learn how to use it to write a mandelbrot in
1-line :-) Haskell should be a doddle - at least you stand to pick up bits
and pieces based on English, rather than just almost random single
characters for everything!
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>> The hard thing about writing a tutorial is that by the time you're
>> experienced enough to be an authority on the language, half the stuff
>> that trips newbies up is so utterly "obvious" to you that you forget
>> to even mention it.
>
> Exactly.
This is one of the hardest things about technical writing - and one of
the things I most often see done wrong. Search around the Internet a
bit, and you'll find Haskellers writing things like "a monad is just a
monoid in the category of endofunctors - what's the problem?" like
normal human beings have any clue WTF that means. (I have *some* clue,
but the category theory definition of a monad doesn't even begin to
explain why it's useful in computer programming.)
>> In truth, if you're using tuples bigger than about three or four
>> elements, you really ought to define a custom type for this stuff
>> instead of relying on tuples.
>
> OK. I take it a custom type is a bit like a struct in C-type languages,
> in that it can hold any number of different types?
It can be. The neat thing about Haskell is that it has "algebraic data
types". These can be like a struct or a record or whatever you want to
call it. Or they can be like an enumeration. OR THEY CAN BE BOTH!
(I guess this is what C would call a "union". But it's type-safe. You
cannot accidentally access a field that does not exist. Or use the wrong
field type.)
>> I can talk some more if you want?
>
> Once you know a few basic things I think it's best to move on to simple
> examples (small multi-line samples, not those artificial ones needed for
> the web).
Some things are easiest to explain by example. Like, some concepts are
so extremely abstract that without an example, it's difficult to
comprehend what's going on. Other things are quite tricky to demonstrate
with examples; you end up needing to use examples which are either
extremely complex, or very contrived. In depends what you're trying to
demonstrate.
I think for general familiarity with a programming language, lots of
small examples is probably the way forward.
>> > let factorial n = product [1..n]
>> > let choose n k = (factorial n) / (factorial k * factorial (n - k))
>> > choose 3 5
>
> Exactly. Even I can work out from that one how functions are defined
> now :-)
Well, yeah, there is that too. ;-)
> BTW what would happen if I typed "choose 3 (-5)" instead?
Well, it would execute
(factorial 3) / (factorial (-5) * factorial (3 + 5))
Given the above definition of factorial,
factorial (-5) = product [1..-5] = product [] = 1
Which I'm guessing means that choose gives the wrong answer.
(I also just realised that that should probably be integer division,
since choose is only supposed to work with integer inputs and yield
integer outputs...)
> I'm just trying to think of a subject for a good example ... how about a
> card game? Firstly how would you represent the cards (tuple or custom
> type for the suit/value?).
At the most basic level, you can always use tuples and code numbers. But
why do that, when the language has extensive support for a better way?
The idiomatic way would probably be to do something like
data Suit = Diamonds | Hearts | Clubs | Spades
data Number =
Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten |
Jack | Queen | King
data Card = Card Suit Number
So Suit and Number are enumerations, and Card is a record with two
fields. The record syntax is probably the most perplexing part. I'm
using positional fields - which is common when there are very few fields
- but you can also use named fields:
data Card = Card {suit :: Suit, numer :: Numer}
I should probably have thrown it into the tutorial, but "thing :: type"
is how you write explicit type signatures in Haskell. And you can do
> 5 :: Int
> 5 :: Double
> 5 :: Rational
> 5 :: Word64
> 5 :: Complex Float
Unlike Java, "5" can represent any possible numeric type. Usually
Haskell deduces which type you want automatically (or makes it
polymorphic). The web interpretter seems to default to
arbitrary-precision integers - which is reasonable.
(Similarly, "5.3" is any fractional type - Float, Double, Rational, etc.)
> Then I assume you would just create a list
> of "Cards" to represent a group of cards that someone was holding (say 5
> cards).
Sure, that'll work. Other representations are possible.
> How would you then check to see if any two cards in that list
> were the same value, or all the same suit, etc?
By default, you can't compare values. That's because it's possible to
define how to do comparison. (E.g., your data structure might have
several equivilent representations for the same thing, and you want
these different representations to be considered "the same".) However,
in the common case that you just want to do a vanilla structural
comparison, you just add "deriving Eq" to the end of the type definition.
Once you do that, you can write things like Hearts == Clubs (which is
obviously going to give you "false"). (By a similar arrangement, you can
auto-define comparison as well. The compiler uses the order you listed
the values in. Might be useful for deciding which card is "higher".)
Now, if you've used named fields, then "suit" and "number" are not only
field names, but also implicitly-defined functions that take a Card and
return the appropriate field. So "suit card1" tells you the suit of
whatever is in card1.
Are all the cards the same suit?
length (group (map suit cards)) == 1
(This could be inefficient if "cards" is huge, but for a hand of 5 or 7
cards, it should be fine.)
First, you extract a list of suits. Then you group this list into a list
of sublists, such that each sublist contains only identical suits. If
*all* the suits are the same, clearly you will have exactly 1 sublist.
(Equally clearly, if you have 1000 cards, alternating between Clubs and
Spades, you will get a list with 1000 1-element sublists, which length
will have to completely traverse before giving you an answer.)
You can use pattern matching to solve the inefficiency, but due to the
limitations of the web interpretter, it's bloody awkward to demonstrate
pattern matching. So I haven't shown that to you yet.
>> Are you actually interested enough to want to play with it more?
>
> I was interested enough in J to learn how to use it to write a
> mandelbrot in 1-line :-) Haskell should be a doddle - at least you
> stand to pick up bits and pieces based on English, rather than just
> almost random single characters for everything!
LOL! OK, well if you put it that way...
Although even Haskell has a few "interesting" symbol choices. ">>="
leaps to mind, but you might also mention "$", "&&&", ":" and so on.
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>> BTW what would happen if I typed "choose 3 (-5)" instead?
>
> Well, it would execute
>
> (factorial 3) / (factorial (-5) * factorial (3 + 5))
>
> Given the above definition of factorial,
>
> factorial (-5) = product [1..-5] = product [] = 1
>
> Which I'm guessing means that choose gives the wrong answer.
So what's the preferred way to cope with runtime "errors" like that? (I
don't know if there is a technical term for such kind of errors that don't
actually make an error, if you know what I mean). I suspect in a lot of
case some very low nested function detects an error, and needs to signify
this to a much higher level calling function (eg a "ParseFile" or
"ParseLine" function or something).
> So Suit and Number are enumerations, and Card is a record with two fields.
> The record syntax is probably the most perplexing part. I'm using
> positional fields - which is common when there are very few fields - but
> you can also use named fields:
>
> data Card = Card {suit :: Suit, numer :: Numer}
OK, so I could also do something like:
data myRecord = myRecord {age :: Int, height :: Double, weight :: Double}
> Are all the cards the same suit?
> length (group (map suit cards)) == 1
> (This could be inefficient if "cards" is huge, but for a hand of 5 or 7
> cards, it should be fine.)
>
> First, you extract a list of suits.
So in my example, if I had a list of myRecords, then "age MyList" would
return a list of ages?
> Then you group this list into a list of sublists, such that each sublist
> contains only identical suits.
What does the "map" do? Clearly I need to download the proper
interpretter/compiler to answer these questions myself with a bit of playing
around :-)
OK so after Googling Haskell, implementations, GHC, go to the GHC page,
latest release, "STOP!" they recommend I get the Haskell Platorm instead, oh
goody a .exe installer link :-) I'll let you know in 54 MB time how I get
on...
> LOL! OK, well if you put it that way...
Thing is it's very rare that I find myself wanting to write something that
*I* think would be easier/faster with a language like Haskell. But as I
don't really know much about the language, I'm not a very good judge.
You're also quite good a writing tutorials, you should definitely write a
book. Haskell for Imperative Programmers. :-)
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>>> BTW what would happen if I typed "choose 3 (-5)" instead?
>>
>> Well, it would execute
>>
>> (factorial 3) / (factorial (-5) * factorial (3 + 5))
>>
>> Given the above definition of factorial,
>>
>> factorial (-5) = product [1..-5] = product [] = 1
>>
>> Which I'm guessing means that choose gives the wrong answer.
>
> So what's the preferred way to cope with runtime "errors" like that? (I
> don't know if there is a technical term for such kind of errors that
> don't actually make an error, if you know what I mean).
The correct thing to do here would be
choose n k
| n < 0 = error "choose: negative n"
| k < 0 = error "choose: negative k"
| k > n = 0 -- According to Wikipedia.
| otherwise = (factorial n) `div` (factorial k * factorial (n - k))
(You would also define it to accept only unsigned integers as arguments,
which neatly does away with most of the error checking.)
> I suspect in a
> lot of case some very low nested function detects an error, and needs to
> signify this to a much higher level calling function (eg a "ParseFile"
> or "ParseLine" function or something).
The standard idiom in Haskell is this:
- If it's a program bug, use the "error" function. In particular, if a
function is called with invalid arguments, this is probably a bug in the
caller.
- If it's a routine, expected occurrance, return a data structure which
indicates success or failure. For example, a parse failure is totally to
be expected, so we need to explicitly handle it. The error is returned
"as data". (This is where the "error monad" comes in...)
The "error" function, technically speaking, throws an exception. Other
actions (generally only I/O operations) may also throw exceptions, and
Haskell has the usual exception-catching machinery. (Although things
like "catch" are library functions, not language constructs.)
> OK, so I could also do something like:
>
> data myRecord = myRecord {age :: Int, height :: Double, weight :: Double}
Yes, that'll work.
(Actually, I'm lying. The type name MUST begin with a capital letter.
This IS enforced by the compiler.)
The catch: Each field name is also a function name. Therefore, no two
types can have fields with the same name. (At least, not in the same
scope. Obviously types defined in different files can.) This restriction
becomes quite tedious at times.
>> Are all the cards the same suit?
>> length (group (map suit cards)) == 1
>> (This could be inefficient if "cards" is huge, but for a hand of 5 or
>> 7 cards, it should be fine.)
>>
>> First, you extract a list of suits.
>
> So in my example, if I had a list of myRecords, then "age MyList" would
> return a list of ages?
No. "map age mylist" would though. You need "map" to apply "age" to
every element of the list.
(And variable names MUST begin with a lowercase letter. The compiler
will complain if they don't.)
> What does the "map" do?
See above. ;-)
> OK so after Googling Haskell, implementations, GHC, go to the GHC page,
> latest release, "STOP!" they recommend I get the Haskell Platorm
> instead, oh goody a .exe installer link :-) I'll let you know in 54 MB
> time how I get on...
Yes, it's... a tad large. It *does* contain a complete copy of GCC, gas,
ld, perl.exe (?!) and a big suite of precompiled libraries, with HTML
documentation, a profiler, a debugger...
Background: Installing GHC used to give you a complete environment. But
the GHC guys don't want to maintain a complete environment, they want to
maintain GHC. So now the Haskell Platform exists, and the HP guys work
on HP, so the GHC guys can focus on just GHC. The idea is that HP will
grow to provide more stuff. Currently there's not actually much
difference. Follow all that?
Oh, and yes, GHC has *always* been big. You just want to play with
Haskell, but what you're downloading has all sorts of bells and whistles
that you won't use. There's a powerful parser library, cutting-edge
parallel processing technology, a complete OpenGL binding, POSIX
bindings, cross-platform filename processing... need I continue?
> Thing is it's very rare that I find myself wanting to write something
> that *I* think would be easier/faster with a language like Haskell.
Well, yes, it completely depends on the kind of code you want to write.
If you want to write, say, a computer game, Haskell probably isn't the
best choice. In C or C++, you can probably just download a game engine
and start coding. In Haskell... there's an OpenGL binding? I guess that
helps? But it's going to take a loooong time to get to a full-featured
game from there!
Personally, I tend to write crazy algorithm stuff. Haskell is great for
that, even if performance is a little unpredictable.
> But
> as I don't really know much about the language, I'm not a very good judge.
Well, yeah. That's kind of what's exciting about the web interpretter,
unfinished as it is. Now anybody can play with Haskell and do basic
stuff without... waiting for a 60+ MB download. ;-)
> You're also quite good a writing tutorials, you should definitely write
> a book. Haskell for Imperative Programmers. :-)
Heh. I've attempted to write *several* Haskell introductions. Roughly
once every 2 months I start writing a new one. They never get finished.
Trouble is, lots of Haskell's features depend on Haskell's other
features, so... where do you start? Which thing do you explain first? I
write an example, and then I go "oh crap, that uses X, I haven't even
mentioned that yet. Bugger!"
It's not complicated, it's just difficult to decide where to begin.
But thanks for your encouragement. Go take a look at the other thing I
just posted. ;-)
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Invisible wrote:
> Of course, it's still alpha quality.
Strike a light... It seems the bit "alpha" sticker has gone. And the
command history seems to work properly now. Neat...
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Invisible wrote:
> Yes, it's... a tad large. It *does* contain a complete copy of GCC, gas,
> ld, perl.exe (?!) and a big suite of precompiled libraries, with HTML
> documentation, a profiler, a debugger...
I guess it's a bit like downloading and installing VisualStudio just so
you can write a Hello World program in C++... It'll do it, but it's kind
of overkill. ;-)
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> data Suit = Diamonds | Hearts | Clubs | Spades
>
> data Number =
> Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten |
> Jack | Queen | King
>
> data Card = Card Suit Number
I get "parse error on input 'data'" for all those.
Guess I'm missing some #include or whatever it's called.
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Invisible <voi### [at] dev null> wrote:
> Invisible wrote:
> > Of course, it's still alpha quality.
> Strike a light... It seems the bit "alpha" sticker has gone. And the
> command history seems to work properly now. Neat...
Now it's only v0.01 beta?-)
--
- Warp
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scott wrote:
>> data Suit = Diamonds | Hearts | Clubs | Spades
>>
>> data Number =
>> Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten |
>> Jack | Queen | King
>>
>> data Card = Card Suit Number
>
> I get "parse error on input 'data'" for all those.
>
> Guess I'm missing some #include or whatever it's called.
Which way are you trying to execute this?
If you're trying to enter them interactively, don't. It doesn't work. ;-)
The GHC interactive interpretter ("GHCi") allows you to define variables
and use them later, but doesn't allow any other kinds of definitions
interactively.
If you write this stuff in a file and save it as, say, "Cards.hs", you
should be able to double-click that file and have GHCi automatically
open up and load it. You can then write expressions involving these types.
(Probably the next problem you'll trip over is that the types aren't
printable...)
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>> Strike a light... It seems the bit "alpha" sticker has gone. And the
>> command history seems to work properly now. Neat...
>
> Now it's only v0.01 beta?-)
But that's the wonderful thing about OSS, isn't it? It never actually
reaches release 1.0. :-D
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> But that's the wonderful thing about OSS, isn't it? It never actually
> reaches release 1.0. :-D
Just like a progress bar in Windows:
ProgressBar1.Value = 1 - exp(time)
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Invisible wrote:
> Strike a light... It seems the bit "alpha" sticker has gone.
Technically, if it was available for download, it's already beta. But I
guess that nomenclature already went out the window a few years ago.
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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scott wrote:
>> But that's the wonderful thing about OSS, isn't it? It never actually
>> reaches release 1.0. :-D
>
> Just like a progress bar in Windows:
>
> ProgressBar1.Value = 1 - exp(time)
Hey, at least OSS version numbers don't (usually) go backwards. :-P
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>> ProgressBar1.Value = 1 - exp(time)
>
> Hey, at least OSS version numbers don't (usually) go backwards. :-P
Jsut checking you were awake :-)
Of course that should be 1-exp(-time)
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scott wrote:
>>> ProgressBar1.Value = 1 - exp(time)
>>
>> Hey, at least OSS version numbers don't (usually) go backwards. :-P
>
> Jsut checking you were awake :-)
>
> Of course that should be 1-exp(-time)
LOL! I didn't even notice that...
But Windows progress bars do, in fact, go both forwards and backwards,
seemingly at random. ;-)
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> But Windows progress bars do, in fact, go both forwards and backwards,
> seemingly at random. ;-)
The best is when it gets to 99 or 100% and then *starts again* at 0% !
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scott wrote:
>> But Windows progress bars do, in fact, go both forwards and backwards,
>> seemingly at random. ;-)
>
> The best is when it gets to 99 or 100% and then *starts again* at 0% !
Or when it goes from 0% to 100% in about 0.1 seconds, and then sits at
100% for 25 minutes... Yeah, that's real useful, guys.
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scott <sco### [at] scottcom> wrote:
> > But Windows progress bars do, in fact, go both forwards and backwards,
> > seemingly at random. ;-)
> The best is when it gets to 99 or 100% and then *starts again* at 0% !
There actually are some programmers out there (even pros) who have actually
not understood the idea of the progress bar. They seem to think it's something
akin go the spinning wheel which is often used when the total time is unknown
(ie. the spinning wheel simply indicates that something is being done, but
the program has no way of knowing how much will be done).
Other programmers (sadly a lot of them) understand the purpose of the
progress bar, but they don't understand how it should be used so that it's
actually informative to the user. The most typical error is that if the
task done by the program consists of dozens of sub-tasks (such as eg.
installing a dozen of files), the progress bar is used to indicate the
progress of each individual subtask instead of using it to indicate the
overall progress. Thus the progress bar becomes useless (and more akin to
the spinning wheel).
More competent programmers (or program designers) will put *two* progress
bars in the UI: One to indicate the progress of the sub-task and the other
to indicate overall progress. That's a whole lot more informative (although
the sub-task progress bar is not so useful if each sub-task is very fast to
perform, but more useful if they are slowish operations).
Of course even then there's room for incompetence: For example, a progress
bar that updates too rarely and makes big jumps, or which pauses for too
long at the beginning or the end (nothing is more confusing than having
the overall progress bar at 100%, but the program still doing things for
several tens of seconds). Of course depending on the task in hand it may
be quite difficult programmatically to update the progress bar smoothly,
but still...
--
- Warp
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Warp wrote:
> There actually are some programmers out there (even pros) who have actually
> not understood the idea of the progress bar. They seem to think it's something
> akin go the spinning wheel which is often used when the total time is unknown
Yes, some programs just have some kind of wheel that lets you know that
it's doing *something*, even if it can't estimate how long it will take.
Which is fine - the problem comes when they make it look like it's a
"progress" indication, when in fact it isn't.
> The most typical error is that if the
> task done by the program consists of dozens of sub-tasks
> the progress bar is used to indicate the
> progress of each individual subtask instead of using it to indicate the
> overall progress. Thus the progress bar becomes useless.
Indeed.
> More competent programmers (or program designers) will put *two* progress
> bars in the UI: One to indicate the progress of the sub-task and the other
> to indicate overall progress. That's a whole lot more informative.
Yes. Although it still sometimes has the problem that some subtasks are
extremely fast, while others are insanely slow. Still, it does at least
provide a monotonically-increasing bar which fills up when the task is
done. It's just harder to estimate time remaining.
(Don't even get me started on "estimated time remaining"...)
> Of course even then there's room for incompetence: For example, a progress
> bar that updates too rarely and makes big jumps, or which pauses for too
> long at the beginning or the end.
Yeah, I hate that!
> Of course depending on the task in hand it may
> be quite difficult programmatically to update the progress bar smoothly,
> but still...
If you can't estimate progress in a meaningful way, don't pretend that
you can. ;-)
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Invisible <voi### [at] devnull> wrote:
> If you can't estimate progress in a meaningful way, don't pretend that
> you can. ;-)
I actually had that problem with a puzzle solver I had to make for our
first iPhone game. (There's a glimpse of it here:
http://gamesandinnovation.com/2010/02/10/hopido-design-post-mortem/ )
The problem was that when the puzzle was large enough, the progress
would often advance like
0% -> 0.000001% -> 0.000002% -> ... -> 0.000158% -> 2.026041% -> 2.026042%
-> 5.0% -> 80.0% -> 100%, or similar. Just putting the percentage on
screen was not informative at all, no matter what I tried. (Often the
percentage would look like the program had halted completely because it
changed only so little that it would have required like 8 decimals to
show the change.)
What I ended up doing instead was to show the current search path as
lines of changing colors over the puzzle, updating a few times per second.
Doesn't tell you much about the progress, but shows clearly that the
program is doing something.
--
- Warp
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Warp wrote:
> Invisible <voi### [at] devnull> wrote:
>> If you can't estimate progress in a meaningful way, don't pretend that
>> you can. ;-)
>
> What I ended up doing instead was to show the current search path as
> lines of changing colors over the puzzle, updating a few times per second.
> Doesn't tell you much about the progress, but shows clearly that the
> program is doing something.
Right. And IMHO, that would be a correct solution. It makes it clear
that something is happening, without giving the user inaccurate progress
indications.
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Warp wrote:
> (ie. the spinning wheel simply indicates that something is being done, but
> the program has no way of knowing how much will be done).
Not in Windows! :-) Try turning of the remote computer while you're
copying files to it with Explorer. Do you think those fluttering pages stop
flying from one folder to the other? NooooOOOoooo...
> the overall progress bar at 100%, but the program still doing things for
> several tens of seconds).
I will say I've seen this happen when the flush() call at the end actually
takes 30 to 90 seconds. Once you figure out what's going on, it's not that
bad, tho.
--
Darren New, San Diego CA, USA (PST)
The question in today's corporate environment is not
so much "what color is your parachute?" as it is
"what color is your nose?"
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scott wrote:
> Clearly I need to download the proper
> interpretter/compiler to answer these questions myself with a bit of
> playing around :-)
>
> OK so after Googling Haskell, implementations, GHC, go to the GHC page,
> latest release, "STOP!" they recommend I get the Haskell Platorm
> instead, oh goody a .exe installer link :-) I'll let you know in 54 MB
> time how I get on...
...so how did you get on?
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> ...so how did you get on?
Other things came up ... I'm easily distracted...
Specifically, I wrote a simple graphical n-body gravity simulator (I *love*
C# and XNA) to explain a couple of things to my girlfriend, I've been
working on the GUI for my program to download and display tracks from my GPS
unit, and the Formula 1 season started :-)
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scott wrote:
>> ...so how did you get on?
>
> Other things came up ... I'm easily distracted...
Heh, figures. ;-)
> Specifically, I wrote a simple graphical n-body gravity simulator (I
> *love* C# and XNA) to explain a couple of things to my girlfriend,
That's *advanced*, right there! o_O
I'm pretty sure if I tried to explain the rules of gravitation to a
girl, I wouldn't _have_ a girlfriend for very long...
(So much for "the gravity of love", eh?)
> I've
> been working on the GUI for my program to download and display tracks
> from my GPS unit, and the Formula 1 season started :-)
Oh. Another F1 fan...
Heh, oh well. I was expecting the reply to be "I can't think of anything
that Haskell would be useful for". ;-)
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> I'm pretty sure if I tried to explain the rules of gravitation to a girl,
> I wouldn't _have_ a girlfriend for very long...
She wasn't interested in space before she met me, it's only when she saw
Saturn through the telescope that she thought it was really cool and started
to read a lot about it - she asks lots of questions. But it's a very fine
line to tread, if I try to make things too simple I get the "I'm not dumb -
god who did you date before me" line, but go too far the other way and you
get "ok i don't understand, i give up, don't bother".
> Heh, oh well. I was expecting the reply to be "I can't think of anything
> that Haskell would be useful for". ;-)
Well that too :-) I guess I just don't do much of the sort of stuff Haskell
would be really good for. OOC are there any "classic" examples of real life
stuff Haskell does better than other languages? Might give me some ideas.
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scott wrote:
>> I'm pretty sure if I tried to explain the rules of gravitation to a
>> girl, I wouldn't _have_ a girlfriend for very long...
>
> She wasn't interested in space before she met me, it's only when she saw
> Saturn through the telescope that she thought it was really cool and
> started to read a lot about it - she asks lots of questions. But it's a
> very fine line to tread, if I try to make things too simple I get the
> "I'm not dumb - god who did you date before me" line, but go too far the
> other way and you get "ok i don't understand, i give up, don't bother".
Hmm. Apparently you have a very cool girlfriend...
[I'm not going to continue this train of thought any further. I don't
want to depress myself.]
>> Heh, oh well. I was expecting the reply to be "I can't think of
>> anything that Haskell would be useful for". ;-)
>
> Well that too :-) I guess I just don't do much of the sort of stuff
> Haskell would be really good for. OOC are there any "classic" examples
> of real life stuff Haskell does better than other languages? Might give
> me some ideas.
Parsers, compilers, interpretters, simulations, anything that does
complex algorithmic processing. (I hear it's popular for things like
financial modelling and IC design.)
Hypothetically Haskell _should_ be an excellent choice for things like
cryptography and data compression, but in practise the performance tends
to be suboptimal.
Anything where correctness is more important than performance or even
possibly development cost. (I hear Haskell is sometimes used in
safety-critical fields like avionics - although you don't actually *run*
Haskell on embedded systems, you use it to generate the code that will
be run, with strong guarantees about correctness of the generated code.)
Anything where parallel processing is useful. In theory this is the
Killer Advantage of Haskell; normal Haskell programs consist of about
98% pure code, which is inherantly thread-safe and hypothetically easy
to parallelise. Again, in practise performance is a little too variable
to claim that Haskell is the killer language for compute-bound problems.
As examples of some of the above:
- The Haskell Platform comes with Parsec, an impressive parsing library
which I'm *sure* I must have documented here at least once before.
- GHC itself is written in 98% Haskell. (Stuff like the garbage
collector and the I/O subsystem is written in C.) Think about it - what
do compilers do? They parse stuff, and they transform code trees. These
are both tasks which Haskell is excellent at.
- There's a small cottage industry for writing things like theorum
provers, constraint solvers, neural networks, genetic algorithms and the
like using Haskell. Writing these things typically isn't difficult;
making it efficient is the tricky part. ;-)
- Hell, you may even remember that I wrote a small logic equation solver
a while back. Remember? It could not only join to lists together, but
also UNjoin them in every possible way. ;-) OK, not terribly useful, but
quite satisfying to get it to work...
- I personally have also written ray tracers, fractal generators,
compression and cryptography algorithms, and all sorts of other crazy
stuff. If that's your thing, Haskell is a great way to write code
quickly. (Not necessarily code that runs quickly, sadly.)
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> simulations,
I heard Haskell has OpenGL capability, how easy is that to get running -
then we might be talking :-)
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scott wrote:
> I heard Haskell has OpenGL capability, how easy is that to get running -
> then we might be talking :-)
Heh, OK, it's just taken me the entire morning to figure this out.
(Documented? What do you mean documented? Why would it be documented??)
First: The Haskell Platform includes OpenGL out-of-the-box, so it should
Just Work(tm).
Second: The documentation for OpenGL is missing, which made me spend 20
minutes trying to figure out why it wasn't there. Querying the package
list confirms that OpenGL actually *is* there, it's only the
documentation that's missing. (Thanks, guys...)
Third: I compiled a simple OpenGL program, and it crashed with "cannot
find glut32.dll". I don't know if that's because I don't have a 3D card
on this machine or what. Anyway, it took a while to fix it! YMMV.
Start with this:
module Main where
import Graphics.Rendering.OpenGL
import Graphics.UI.GLUT
myPoints :: [(GLfloat,GLfloat,GLfloat)]
myPoints = map (\k -> (sin(2*pi*k/12),cos(2*pi*k/12),0.0)) [1..12]
main = do
(progname, _) <- getArgsAndInitialize
createWindow "Hello World"
displayCallback $= display
mainLoop
display = do
clear [ColorBuffer]
renderPrimitive Points $ mapM_ (\(x, y, z)->vertex$Vertex3 x y z)
myPoints
flush
Save that lot as Test.hs or something. Then execute
ghc --make Test
That should generate Test.exe. Now run it.
If it whines about not finding glut32.dll, go here:
http://www.transmissionzero.co.uk/software/freeglut-devel/
Download the "FreeGLUT MinGW" package, and unpack glut32.dll from it.
(You can ignore all the other files; you only need the DLL.) Drop the
DLL into your search path or just into the same folder as Test.exe, and
then try and run it.
If it works, you should get a black window with some white dots.
Assuming all of that works, you now just need to learn the whole Haskell
programming language, including how to do I/O, and learn the OpenGL
binding, and then you can do stuff. ;-)
And now if you'll excuse me, I'm going to go file a bunch of tickets
with the bugtracker. :-P
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Invisible <voi### [at] devnull> wrote:
> I compiled a simple OpenGL program, and it crashed with "cannot
> find glut32.dll". I don't know if that's because I don't have a 3D card
> on this machine or what. Anyway, it took a while to fix it! YMMV.
Yup, OpenGL doesn't ship with GLUT (OpenGL Utility Toolkit). You need to
download it separately.
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> myPoints
> flush
>
> Save that lot as Test.hs or something. Then execute
>
> ghc --make Test
I get:
Test.hs:20:0: parse error (possibly incorrect indentation)
I guess something with the indentation is not correct at the end of the .hs
file? What does it need?
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scott wrote:
>> myPoints
>> flush
>>
>> Save that lot as Test.hs or something. Then execute
>>
>> ghc --make Test
>
> I get:
>
> Test.hs:20:0: parse error (possibly incorrect indentation)
>
> I guess something with the indentation is not correct at the end of the
> .hs file? What does it need?
The "myPoints" is supposed to be at the end of the previous line.
Apparently it got wrapped...
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Cool it works! (I had to rename freeglut.dll to glut32.dll)
Now to learn Haskell...
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scott wrote:
> Cool it works! (I had to rename freeglut.dll to glut32.dll)
Oh, yeah, I forgot to mention that part. o_O
> Now to learn Haskell...
LOL! Um, yes. ;-)
You'd probably be best learning the pure part of Haskell first, since
this is the primary objective of Haskell. The impure part of Haskell is
just like any other language - but far, far more wordy. Thing is, OpenGL
is all *about* the impure part, so...
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scott wrote:
> You're also quite good a writing tutorials, you should definitely write
> a book. Haskell for Imperative Programmers. :-)
If I write it, will you help review it?
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>> You're also quite good a writing tutorials, you should definitely write a
>> book. Haskell for Imperative Programmers. :-)
>
> If I write it, will you help review it?
Of course! But I expect 50% of the profits :-D
Also it will be a good excuse to hold off learning Haskell for now, as then
I'll still be a real beginner when you finish the first draft...
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scott wrote:
>>> You're also quite good a writing tutorials, you should definitely
>>> write a book. Haskell for Imperative Programmers. :-)
>>
>> If I write it, will you help review it?
>
> Of course! But I expect 50% of the profits :-D
Sure! I'll give you 50% of £0 if ya want it. ;-)
> Also it will be a good excuse to hold off learning Haskell for now, as
> then I'll still be a real beginner when you finish the first draft...
Hehehe, OK then. :-)
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