tryhaskell.org
I've read through some Haskell material before and tried following tutorials, but I always end up dropping it because I can't think of anything for which I would use it.
I have no problem with functional and functional-esque programming (Erlang, Common Lisp, etc.), but I just can't come up with anything to do in Haskell. :(
EDIT:
Thanks for the great suggestions, everyone! I think I'll do some crpyo-based maths work in Haskell since that is what holds my interest at the moment. :-)
Writing an extension for XMonad may be worthwhile, though. Or learning about its inner workings. (There are some good introductions floating around the net. They put a lot of thought into the types in XMonad.)
http://jonathan.tang.name/files/scheme_in_48/tutorial/overvi...
What do you normally program? Do that, but in Haskell. It's a general-purpose programming language, and there are plenty of libraries to help you in any area.
tl;dr version:
1) It's a pretty language.
2) It's fast.
3) Static typing (and its optional companion "formal verification") rock.
4) Higher-order functions, Algebraic Data Types, Polymorphism and currying are our friends.
5) Exceptions are evil[1], explicit error handling is safer. Finally a language that hands me more convenient way of explicit error handling then:
int result = someCLibraryCall();
if (result == -1) {
fprintf(stderr, "Ach, mein leben!");
exit(EXIT_FAILURE);
}
Arbitrarily changing prefix into infix functions or vice versa:
elem foo list == foo `elem` list
1 + 2 == (+) 1 2
2) For a language as high-level as Haskell, it's FAST. Most of the benchmarks show it pretty consistently in the C/Java performance regions (depending on what you're doing of course).
3) I used to hate static typing and think dynamic typing was the "end all, be all" of typing solutions until I start learning Haskell and suddenly realized I don't hate static typing, but that I hate crappy static typing implementations (Java, C, I'm looking at you).
A bonus of this is that it makes program verification of Haskell programs A LOT easier. During my class on protocol validation the mu-calculus we used for verifying programs was almost trivial to translate to Haskell (and vice versa).
I'm a firm believer in "formal verification" as a means to detect/prevent bugs. And while it might not be an issue for the programming world at large right now[2] (who cares if a word processor or browser crashes), it is important for mission critical software on things like cars, train, airplanes and space shuttles...
As a side note, I still like and use dynamic systems like python, but only for "trivial" code. i.e. quick scripts and stuff where I don't care if it breaks.
4) I find that with Haskell I'm usually working on a much higher level of abstraction then otherwise. Which means I'm spending more time thinking and scribbling on my white board and less time writing code and fixing stupid mistakes.
5) Monads like Maybe and Either allow for various convenient ways to do explicit error handling (there's probably a gazillion more ways to do explicit error handling with monads, but I don't grok them enough yet).
While Haskell still has exceptions they're far more limited and more sparsely used then in other languages.
EDIT: Removed very verbose and vague description of the Maybe monad, examine the example posted by jmillikin below instead.
In conclusion, what's Haskell good for? Well, I find it lends itself pretty well for high-level, abstract, meta sort of programming. The type a lot of people advertise Lisp for. And while Lisp macro's might be a tad more expressive[3], Haskell is most certainly prettier. And as behoofs any (ex-)python programmer I'm convinced readability matters.
All this while still being fast, reasonably good at stopping you from writing bugs and relatively easy to debug. Sounds like good enough reasons to use it, for me.
[1] - I'm not gonna argue "why" here, if you want to know I recommend Googling "Exceptions considered harmful" or "why exceptions suck" and you'll find plenty of in-depth explanations.
[2] - Actually, I do think it is relevant to ALL programmers. But that's more of a personal opinion which not a lot of people seem to share.
[3] - This is a hunch, learning Lisp/Scheme/Clojure is still on my todo list, so I'm not entirely sure if Lisp macro's can do things which are impossible in Haskell or not.
case fileExists "sausage.txt"
True -> putStrLn "Gonna do something with the sausage."
False -> putStrLn "Woop."
if do fileExists "sausage.txt"
then putStrLn "Gonna do something!"
else putStrLn "Woop."
(\case of (Just a) -> a; Nothing -> b)
But Haskell is terse and predictable so one puts up with not having macros.
The Computer Language Shootout Haskell samples are low level, if you read them. Some are high level but many are nearly C, using malloc and such. But this is a good thing; you can write the "tight loops" that you need within Haskell.
Type Haskell expressions in here.
> let x = 10
not an expression: `let x = 10'
I don't need the mtl stuff, but being able to define a basic function would be nice!
http://codepad.org/ will run full programs for you ... I guess that'll continue to be my tool of choice for online Haskell prototyping.
Still, this project does seem promising.
To be honest, if you're at the point where you want that sort of thing, I'd generally suggest simply downloading ghci. ISTM that all these "try it online" things are good for are making the decision of whether you want to bother with that step. (Perhaps I'm spoiled on Linux where downloading GHC or Ruby is hardly any harder than visiting a web page anyhow.)
> let x = 10 ; y = 5 ; in x + y
=> 15
:: (Num t) => tI'll be back to this site in a few weeks to check for progress...
I'd like to be able to tell those jokes without it being like making sounds in a foreign language—that's what I mean by "knowing" Haskell. You can't speak a language until you can make a witty retort in it ;)
That's just computer science -- a bunch of CS stuff is easier to express in Haskell, so we do, but it might need some background.
> here's how to do in Haskell the stuff you already know how to do in Ruby
Try a different tutorial. Maybe: Real World Haskell?
Lets define a simple type, `Maybe', which can store either a value or an absence of value:
data Maybe a = Just a | Nothing
deriving (Show)
clamp :: Ord a => a -> a -> Maybe a
clamp limit x = if x > limit then Nothing else Just x
One way is to just brute-force it:
multiclamp :: Ord a => a -> a -> a -> Maybe a
multiclamp lim1 lim2 x = case clamp lim1 x of
Nothing -> Nothing
Just x' -> clamp lim2 x'
First, take a look at the core Monad operations. I'm going to use `build' and `bind' instead of `return' and `>>=', because they're probably more familiar to most readers.
class Monad m where
build :: a -> m a
bind :: m a -> (a -> m b) -> m b
instance Monad Maybe where
build x = Just x
bind Nothing _ = Nothing
bind (Just x) f = f x
multiclamp2 :: Ord a => a -> a -> a -> Maybe a
multiclamp2 lim1 lim2 x = bind
(clamp lim1 x)
(\x' -> clamp lim2 x')
multiclamp3 :: Ord a => a -> a -> a -> Maybe a
multiclamp3 lim1 lim2 x =
(clamp lim1 x) >>=
(\x' -> clamp lim2 x')
multiclamp4 :: Ord a => a -> a -> a -> Maybe a
multiclamp4 lim1 lim2 x = do
x' <- clamp lim1 x
clamp lim2 x'
For completeness, here's a "cleaner" version, which takes advantage of currying and the combinator `.':
multiclamp5 :: Ord a => a -> a -> a -> Maybe a
multiclamp5 lim1 lim2 = (clamp lim2 =<<) . clamp lim1 is the purpose of the Maybe monad to abstract the concept of "evaluate to Nothing if the previous expression evaluated to nothing?
Also, is 'build'/'return' being used implicity somewhere?
In real code, it's mostly used when the exact monad being used isn't specified. For example, here's a function which runs a monadic computation until it returns `False'. The result is `()', sort of a Haskell void type.
runUntil :: Monad m => m Bool -> m ()
runUntil comp = loop where loop = do
x <- comp
if x then loop else return ()
C# and Python can, sort of, but their syntax and object-oriented semantics make it somewhat difficult. I don't know C# well, and Python needs a lot of magic[1] to get reasonable-looking code.
Here's a Python example. It's not a direct translation, but is close enough that you should be able to figure out the original. I've included the Haskell code in comments.
import abc
# class Monad m where
# build :: a -> m a
# bind :: m a -> (a -> m b) -> m b
class Monad (metaclass = abc.ABCMeta):
@classmethod
def build (cls, x):
raise NotImplementedError
def bind (self, f):
raise NotImplementedError
# data Maybe a = Just a | Nothing
# deriving (Show)
class Maybe (Monad):
def __init__ (self, x):
self.__x = x
def __repr__ (self):
if self is Nothing:
return "Nothing"
return ("Just %r" % self.__x)
# instance Monad Maybe where
# build x = Just x
#
# bind Nothing _ = Nothing
# bind (Just x) f = f x
@classmethod
def build (cls, x):
return Just (x)
def bind (self, f):
if self is Nothing:
return Nothing
return f (self.__x)
def Just (x):
return Maybe (x)
Nothing = Maybe (None)
# clamp limit x = if x > limit then Nothing else Just x
def clamp (limit, x):
if x > limit:
return Nothing
return Just (x)
# multiclamp2 lim1 lim2 x = bind
# (clamp lim1 x)
# (\x' -> clamp lim2 x')
def multiclamp2 (lim1, lim2, x):
return clamp (lim1, x).bind (lambda x2: clamp (lim2, x2))
http://lamp.epfl.ch/~emir/bqbase/2005/01/20/monad.html has an explanation from scala's perspective. I can understand the first part when the author describes a monad as an object with a polymorphic type, a map function, a flatten function, and a singleton constructor function. Everything they said beyond that goes right over my head.
For me at least, the mathematics line-noise is a bigger issue, and it made more sense once I ignored it. As a programming construct, monads seem quite understandable to me, even clever. But many of the explanations, especially earlier ones, started out with a type-theoretic explanation with links to category theory...
In particular, I was under the misimpression for years, because of it being presented that way, that monads were essentially of interest only for people who were hung up on type-checking: that they were a solution to how to make the type-checking go through on a certain class of problems, by lifting an underlying type into the Monadic type and getting it back out again. As someone who mainly uses dynamically typed languages, this didn't seem of interest, since in Lisp, I don't care if the type-checking goes through or not. It was probably years later that I figured out they had some non-type-related programming benefit.
Inspired by this article. and in a fit of temporary insanity, I implemented a simple monad protocol and the Maybe and List monads in Clojure. It's not fit for use, but it was fun to write.
It may not be immediately evident what is being composed there, but if you look at lift, it breaks the multi-argument "+" operator into several partial applications and composes each of those under the given monad, giving interesting results. This was necessary to maintain support for a variable number of arguments.
Note that the code uses features only available in the git master branch, so you will need a snapshot build to run it.
multiclamp5 :: Ord a => a -> a -> a -> Maybe a
multiclamp5 lim1 lim2 = Kleisli (clamp lim1) >>> Kleisli (clamp lim2)
multiclamp = clamp >=> return clamp
allclamp lims x = foldM (flip clamp) x lims
(>=>) :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c)
(>=>) c1 c2 a = (c1 a) >>= c2
Pretty cool app. A tutorial to go along with it would do wonders.
> reverse [1..]
Time limit exceeded:m + Data.List intersperse '.' "hello"
And anything else you'd need if you want to use stuff other than just the Prelude.
:m + Data.List intersperse '.' "hello"
And anything else you'd need if you want to use stuff other than just the Prelude.
:m + Data.List intersperse '.' "hello"
And anything else you'd need if you want to use stuff other than just the Prelude.
:m + Data.List intersperse '.' "hello"
And anything else you'd need if you want to use stuff other than just the Prelude.