[Haskell-cafe] Data.List / Map: simple serialization?

[Dmitri O.Kondratiev]

Thanks for the excellent explanation! :

On Fri, Jun 10, 2011 at 4:49 PM, Daniel Fischer <
daniel.is.fischer at googlemail.com> wrote:

> On Friday 10 June 2011, 14:25:59, Dmitri O.Kondratiev wrote:
> > Two questions:
> > 1) Why to use 'fmap' at all if a complete file is read in a single line
> > of text?
>
> Well, it's a matter of taste whether to write
>
>    foo <- fmap read (readFile "bar")
>    stuffWithFoo
>
> or
>
>    text <- readFile "bar"
>    let foo = read text
>    stuffWithFoo
>
> The former saves one line of code (big deal).
>
> >
> > 2) Trying to use 'fmap' illustrates 1) producing an error (see below):
> > main = do
> >      let xss = [[1,2,3],[4,5,6],[7,8],[9]]
> >      writeFile "output.txt" (show xss)
> >      xss2 <- fmap read (readFile "output.txt") :: [[Int]]
>
> That type signature doesn't refer to xss2, but to the action to the right
> of the "<-", `fmap read (readFile "output.txt")'
>
> readFile "output.txt" :: IO String
>
> so
>
> fmap foo (readFile "output.txt") :: IO bar
>
> supposing
>
> foo :: String -> bar
>
> You want read at the type `String -> [[Int]]', so the signature has to be
>
>    xss2 <- fmap read (readFile "output.txt") :: IO [[Int]]
>
> >      print xss2
> >
> > == Error:
> >  Couldn't match expected type `[String]'
> >              with actual type `IO String'
> >  In the return type of a call of `readFile'
> >  In the second argument of `fmap', namely `(readFile "output.txt")'
> >  In a stmt of a 'do' expression:
> >      xss2 <- fmap read (readFile "output.txt") :: [[Int]]
>
> Looking at the line
>
>    xss2 <- fmap read someStuff :: [[Int]]
>
> the compiler sees that
>
> fmap read someStuff should have type [[Int]]
>
> Now, fmap :: Functor f => (a -> b) -> f a -> f b
>
> and [] is a Functor, so the fmap here is map, hence
>
> map read someStuff :: [[Int]]
>
> means
>
> someStuff :: [String]
>
> That's the expected type of (readFile "output.txt"), but the actual type is
> of course IO String, which is the reported error.
>
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