对字符串使用haskell map函数

Question

I'm trying to use the map function in haskell

I've got this:

lexi :: String -> [[String]]
lexi x = map (words.lines) x

I want to be able to put a string in to x, so it can be run like this

lexi ("string here")

But get the error

Couldn't match type ‘[Char]’ with ‘Char’
Expected type: String -> String
  Actual type: String -> [String]
In the second argument of ‘(.)’, namely ‘lines’
In the first argument of ‘map’, namely ‘(words . lines)’

Couldn't match type ‘Char’ with ‘[Char]’
Expected type: [String]
  Actual type: String
In the second argument of ‘map’, namely ‘x’
In the expression: map (words . lines) x

I know that if I use

lexi = map (words.lines) 

it works fine when I run lexi ("string here") , but need the variable to use later on

Could some please explain why this doesn't work and how to fix it?

Thank you :)

Answer 1

This answer refers to an old version of the question .

So let's get this quite clear (please always add the type signature of all functions you're talking about!)

function :: Char -> [String]

Well, then the type of map function is [Char] -> [[String]] , ie String -> [[String]] . But you want the result to be only [String] , not a triply-nested list. You probably want to join the lists of two levels together; in general the function to use for list-joining purposes is concat (or more generally, join from the Control.Monad module). In this case, you have two different options:

• Join the result of each call to function . Ie, instead of mapping function alone, you map join . function join . function , which has type Char -> String . Mapping that has the desired type String -> [String] .

   lexi = map $ join . function 

• Join the final result of the mapping, ie lexi = join . map function lexi = join . map function . This combination of mapping and joining the results is actually an extremely common task, it has a special operator: monadic bind!

   lexi x = x >>= function 

New version

So we know that

words, lines :: String -> [String]

thus words . lines words . lines can not work, because you're trying to feed a list of strings into a function that only accepts a single string. What you can of course do though is map words over the result, ie map words . lines map words . lines . That has in fact the correct signature and probably does just what you want.