[Haskell-begin] some basic syntax questions

[Felipe Lessa]

On Fri, Jul 25, 2008 at 4:45 PM, Anatoly Vorobey <avorobey at gmail.com> wrote:
> 1. I initially tried putStrLn("Hello, " ++ show $ sumIt args), but that
> didn't compile. Why not?

You see,

Prelude> :i (++)
(++) :: [a] -> [a] -> [a] 	-- Defined in GHC.Base
infixr 5 ++
Prelude> :i ($)
($) :: (a -> b) -> a -> b 	-- Defined in GHC.Base
infixr 0 $

so

   "Hello, " ++ show $ sumIt args

is parsed as

   ("Hello, " ++ show) $ sumIt args

which is an obvious error.

> 2. I initially tried
>
> where sumIt = sum $ map read
>
> (the "point-free" style, I believe it's called?) but that didn't compile.
> Why not? A friend suggested
>
> where sumIt = sum . map read
>
> and that does work; I guess my real problem, then, is that I don't really
> understand the difference
> between the two and why the former doesn't work.

It helps to see the types and the definitions

   ($) :: (a -> b) -> a -> b
   f $ x = f x

   (.) :: (b -> c) -> (a -> b) -> (a -> c)
   f . g = \x -> f (g x)

so ($) applies an argument, while (.) composes functions. For example,

   sumIt x = sum (map read x)

so

   sumIt x = sum $ map read x

by ($)'s definition. On the other hand

   sum $ map read    is    sum (map read)   [by ($)'s definition again]

which is, again, a mistake. However, we may write sumIt as

   sumIt = \x -> sum (map read x)

so it becames clear that

   sumIt = sum . map read



Did I help at all? =)

-- 
Felipe.