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Re: [Haskell-cafe] local loops
by Jon Cast 14 Dec '04

14 Dec '04

Per Larsson <per(a)L4i.se> wrote: > I often use local loops in monadic code, e.g. > > main = do > ... > let loop = do > ... > if cond then loop else return () > loop > > It seems that I can encode this idiom slightly more concise with the 'fix' > operator (from Control.Monad.Fix), i.e. > > main = do > ... > fix (\loop -> do > ... > if ...) > > But is it really semantically equivalent? Is it as efficient? > > Per It is semantically equivalent. I don't know if it is as efficient; looking at the source code, I would expect GHC to inline fix anywhere it occurs, which yields your original version. Jonathan Cast

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The difference between ($) and application
by Derek Elkins 14 Dec '04

14 Dec '04

> Derek Elkins wrote: > >Andrew Pimlott wrote: > >>I think this post should go under the heading "($) considered > >>harmful". I've been bitten by this, and I never use ($) anymore in > >>place of parentheses because it's too tempting to think of it as > >>syntax. > > > >I find this position ridiculous. [...] If you ever make a mistake > >one way the type checker will tell you. > > For what it's worth, this is not true in the presence of implicit > parameters: > > f :: ((?p :: Int) => Int) -> Int > f g = let ?p = 1 in g > > x,y :: Int > x = let ?p = 2 in (f ?p) ==> 1 > y = let ?p = 2 in (f $ ?p) ==> 2 > > I wouldn't mind ($) being magical, along the lines of the magical > runST in a rank-1 type system. It would be nice to be able to use it > in patterns too. > > -- Ben Moral of the story, and if I'm not mistaken, in your opinion, is not to use implicit parameters (as currently formulated). I.e. the consensus seems to be that this is implicit parameters' fault not ($)'s. Personally, I would mind ($) being magical. One of the nice things about Haskell is there is practically no magic. The last thing I want is Haskerl (http://www.dcs.gla.ac.uk/~partain/haskerl.html) runST simply had a rank-2 type that is not expressible in Haskell 98, it seems getting ($) to "work" would be qualitatively different.

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Sound library?
by Jason Bailey 14 Dec '04

14 Dec '04

Would anyone know of packages out there for Haskell that support mp3's or ogg files? thanks Jason

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special term describing f :: (Monad m) => (a -> m b)?
by Derek Elkins 14 Dec '04

14 Dec '04

> What is a function of the followning type called: > > f :: (Monad m) => (a -> m b) > > Is there a special term describing such a function (a function into a > monad)? > > For f in > a >>= f > is en example. > > Need it for an article/report. > > Regards/Henning Well, formally, it's called a Kleisli arrow (or morphism). A less formal term commonly used for values of type a -> m b and m b, is 'computation' or'action' (though 'action' is usually used for the specific case of IO). In "Notions of Computation and Monads" Moggi calls them 'commands'.

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Named function fields vs. type classes
by Derek Elkins 14 Dec '04

14 Dec '04

> > On the other hand, it's difficult or impossible to make a list of a > > bunch of different types of things that have nothing in common save > > being members of the class. > > I've recently been playing with making, for each class C, a > "interface" datatype IC (appropriately universally and existentially > qualified so as to include a dictionary for class C), and then making > this IC an instance of class C. Then I can wrap any instance of C up > in an IC, and make a list of those. > > The casts get a bit annoying, though; the compiler can't figure out > that this IC is in some sense the maximum type in class C, and so > can't resolve things like > > f :: MyClass a => [a] -> b > f = ... > > upcast :: MyClass a => a -> IMyClass -- usually defined as an > instance of class Cast upcast x = IMyClass x > > f [upcast a, upcast b] -- yields type error > > Instead, you have to redefine f as follows: > > f' :: [IMyClass] -> b > > which is a bit annoying. > > HTH. > > --KW 8-) Not surprisingly, the wiki (http://www.haskell.org/hawiki/ExistentialTypes) has some discussion about this as well, though not too much to add to what has been said.

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The difference between ($) and application
by oleg＠pobox.com 14 Dec '04

14 Dec '04

The operator ($) is often considered an application operator of a lower precedence. Modulo precedence, there seem to be no difference between ($) and `the white space', and so one can quickly get used to treat these operators as being semantically the same. However, they are not the same in all circumstances. I'd like to observe an important case where replacing the application with ($) in a fully-parenthesized expression can lead to a type error. > {-# OPTIONS -fglasgow-exts #-} > > module Foo where > > data WR = WR (Int -> Int) > data W = W (forall a. a->a) > > t1 = WR id > t2 = W id We can also write > t1' = WR $ id However, if we try > t2' = W $ id we get an error: /tmp/t1.hs:13: Inferred type is less polymorphic than expected Quantified type variable `a' escapes Expected type: (a -> a) -> b Inferred type: (forall a1. a1 -> a1) -> W In the first argument of `($)', namely `W' In the definition of `t2'': t2' = W $ id Incidentally, Hugs -98 gives a quite bizarre error message ERROR "/tmp/t1.hs":13 - Use of W requires at least 1 argument It didn't complain about "WR $ id"... The reasons for that behavior are obvious: the compiler cannot generalize to higher-ranked types when instantiating the type of ($). It makes a difference that the application is a built-in construction, whereas ($) is just a regular function.

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special term describing f :: (Monad m) => (a -> m b) ?
by Henning Sato von Rosen 14 Dec '04

14 Dec '04

What is a function of the followning type called: f :: (Monad m) => (a -> m b) Is there a special term describing such a function (a function into a monad)? For f in a >>= f is en example. Need it for an article/report. Regards/Henning

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local loops
by Per Larsson 14 Dec '04

14 Dec '04

I often use local loops in monadic code, e.g. main = do ... let loop = do ... if cond then loop else return () loop It seems that I can encode this idiom slightly more concise with the 'fix' operator (from Control.Monad.Fix), i.e. main = do ... fix (\loop -> do ... if ...) But is it really semantically equivalent? Is it as efficient? Per

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Flattening tail recursion?
by GoldPython 13 Dec '04

13 Dec '04

I'm missing something, a functional idiom, whatever. I know there's "length" to count the elements in a list and it works fine, but the function below stack overflows on a large list. countLines [] = 0 countLines (_:ls) = 1 + countLines ls I would have thought that this was tail recursive and would be flattened into iteration by the compiler. Can anyone explain why? Is it because the call is embedded in an expression? And additionally, how can I predict when the flattening will occur? Thanks, Will

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FiniteMapFiniteMap
by Dominic Steinitz 13 Dec '04

13 Dec '04

See also http://www.mail-archive.com/haskell-cafe@haskell.org/msg05809.html Dominic.

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