El vie, 23-07-2010 a las 23:27 -0400, Ronald Guida escribió:

I am trying to figure out how to use GHC's arrow commands, and I found some extremely weird behavior.

CC'ed to ghc-users, because this may be a ghc bug.

In GHC's manual, there is a description of arrow commands, which I don't really understand. http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html#... (Primitive Constructs)

I have two questions: 1. What are arrow commands supposed to do? 2. What is this code supposed to do?

-- start of code --

{-# LANGUAGE Arrows #-} module Main where

import Control.Arrow

foo :: (b -> String) -> ((((b, Int), Float), Double) -> String) -> (b -> String) foo f g b = f b ++ " " ++ g (((b, 8), 1.0), 6.0)

bar :: (t -> String) -> ((Double, Int) -> String) -> t -> String bar f g = proc x -> do (f -< x) `foo` \n m -> g -< (n)

main = do putStrLn $ foo show show 17 putStrLn $ bar show show 17 putStrLn $ foo show show 42 putStrLn $ bar show show 42

-- end of code --

Output from GHCi:

17 (((17,8),1.0),6.0) 17 (6.730326920298707e-306,0) 42 (((42,8),1.0),6.0) 42 (6.730326920298707e-306,0)

Output after compiling with GHC:

17 (((17,8),1.0),6.0) 17 (5.858736684536801e-270,0) 42 (((42,8),1.0),6.0) 42 (5.858736684536801e-270,0)

GHC Version: The Glorious Glasgow Haskell Compilation System, version 6.12.3

This seems to be a bug in ghc. First, let's fix bar to give the full three arguments (Int, Float, Double) to g: bar f g = proc x -> do (f -< x) `foo` (\n m k -> g -< (n,m,k)) ghc infers the type: bar :: (t -> String) -> ((Double, Float, Int) -> String) -> t -> String and we see that the argument order in the second argument to bar is reversed. But the arguments are still given to bar in the order (Int, Float, Double). For example, the 6.0 in foo is interpreted as an Int and outputs a 0 (the first 32 bits in such a small double are zeros). When one varies the numbers in foo, one can see the effects in bar. Can someone from GHC HQ confirm my understanding, or is this just not supposed to work with multiple arguments? Jürgen