[Haskell-beginners] (.) vs ($)

[Mike Meyer]

As is often the case with Haskell, your answer is in the types:

Prelude> :t ($)
($) :: (a -> b) -> a -> b
Prelude> :t (.)
(.) :: (b -> c) -> (a -> b) -> a -> c

So $ takes a function and applies it to a value. . takes two functions and
composes them and applies the result to a value.

So readFirst xs = (readFile.head) xs, or just readFirst = readFile . head.
But readFirst xs = (readFile $ head) xs will also fail, because readFile
doesn't work on objects of type head.

On Fri, Apr 3, 2015 at 9:23 PM, Vale Cofer-Shabica <
vale.cofershabica at gmail.com> wrote:

> Could someone please explain why the commented line fails
> spectacularly while the final line succeeds?
>
> >import System.IO (getContents)
> >import System.Environment (getArgs)
>
> >fileInput :: IO String
> >fileInput = getArgs>>=readFirst where
> >  readFirst :: [FilePath] -> IO String
> >  readFirst [] = System.IO.getContents
> >--readFirst xs = readFile.head xs
> >  readFirst xs = readFile $ head xs
>
>
> I'm particularly confused given the following typings (from ghci):
>
> readFile.head :: [FilePath] -> IO String
> readFile.head [] :: a -> IO String
>
> And this is still stranger:
>
> :type readFile.head ["foo", "bar"]
>
> <interactive>:28:16:
>     Couldn't match expected type `a0 -> FilePath'
>                 with actual type `[Char]'
>     In the expression: "foo"
>     In the first argument of `head', namely `["foo", "bar"]'
>     In the second argument of `(.)', namely `head ["foo", "bar"]'
>
> <interactive>:28:23:
>     Couldn't match expected type `a0 -> FilePath'
>                 with actual type `[Char]'
>     In the expression: "bar"
>     In the first argument of `head', namely `["foo", "bar"]'
>     In the second argument of `(.)', namely `head ["foo", "bar"]'
>
>
> Many thanks in advance,
> vale
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>


On Fri, Apr 3, 2015 at 9:23 PM, Vale Cofer-Shabica <
vale.cofershabica at gmail.com> wrote:

> Could someone please explain why the commented line fails
> spectacularly while the final line succeeds?
>
> >import System.IO (getContents)
> >import System.Environment (getArgs)
>
> >fileInput :: IO String
> >fileInput = getArgs>>=readFirst where
> >  readFirst :: [FilePath] -> IO String
> >  readFirst [] = System.IO.getContents
> >--readFirst xs = readFile.head xs
> >  readFirst xs = readFile $ head xs
>
>
> I'm particularly confused given the following typings (from ghci):
>
> readFile.head :: [FilePath] -> IO String
> readFile.head [] :: a -> IO String
>
> And this is still stranger:
>
> :type readFile.head ["foo", "bar"]
>
> <interactive>:28:16:
>     Couldn't match expected type `a0 -> FilePath'
>                 with actual type `[Char]'
>     In the expression: "foo"
>     In the first argument of `head', namely `["foo", "bar"]'
>     In the second argument of `(.)', namely `head ["foo", "bar"]'
>
> <interactive>:28:23:
>     Couldn't match expected type `a0 -> FilePath'
>                 with actual type `[Char]'
>     In the expression: "bar"
>     In the first argument of `head', namely `["foo", "bar"]'
>     In the second argument of `(.)', namely `head ["foo", "bar"]'
>
>
> Many thanks in advance,
> vale
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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