"Dmitri O.Kondratiev"
xss2 <- read `fmap` readFile "output.txt"
Two questions: 1) Why to use 'fmap' at all if a complete file is read in a single line of text?
Because it's not 'map', it's more generalized. So the argument ('read' here) is applied to whatever is "inside" the second argument ('readFile ...'). Here xss2 <- read `fmap` readFile "output.txt" is equivalent to xss2 <- return . read =<< readFile "output.txt" or tmp <- readFile "output.txt" let xss2 = read tmp
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]] print xss2
fmap read (readFile "output.txt") is of type IO [[Int]], not [[Int]]. -k -- If I haven't seen further, it is by standing in the footprints of giants