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

4 Apr 2015


      Thank you for all the responses! Sumit's point about function
application binding most strongly was the point I was missing. I was
lured by the possibility of writing my function in point-free style as
Mike indicated: readFirst = readFile.head, but ghc complained about
differing numbers of arguments when I included the different case for
the empty list.

Thanks again,
vale


On Sat, Apr 4, 2015 at 1:46 AM, Sumit Sahrawat, Maths & Computing, IIT
(BHU)  wrote:
...
To reiterate what others have said,
readFile . head xs
== readFile . (head xs)     { function application binds strongest }
== (.) readFile (head xs)   { operators are also functions }
The types,
(.)      :: (b -> c) -> (a -> b) -> (a -> c)
   readFile :: FilePath -> IO String
   head xs  :: FilePath
This cannot work as the types don't match. On the other hand, using ($)
instead of (.) will work.
Try writing that out and reasoning with the types on pen and paper, as an
exercise.
If you're interested, there is an excellent post about equational reasoning
here: http://www.haskellforall.com/2013/12/equational-reasoning.html
Enjoy :)
On 4 April 2015 at 08:10, Mike Meyer  wrote:
...
As is often the case with Haskell, your answer is in the types:
Prelude> :t ($)
($) :: (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
 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
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On Fri, Apr 3, 2015 at 9:23 PM, Vale Cofer-Shabica
 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@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
--
Regards
Sumit Sahrawat