Thanks for the excellent explanation! : On Fri, Jun 10, 2011 at 4:49 PM, Daniel Fischer < daniel.is.fisc...@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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