Haskell filter function on strings

Question

Is there a way to use the filter function on Strings, this way:

filter (=="!") "!!some!!_!!string!!"

should output "some_string" (case above). Right now all i get is a type error:

Couldn't match expected type `[Char]' against inferred type `Char'

If i change the second filter argument type to ["!!some!!_!!string!!"], the type error disappears, yet only an empty list is outputted. (Not exactly what i want)

I thought that strings were lists but obviously "!!some!!_!!string!!" isn't regarded as a list but as a char.

Any hints someone ?

kennytm · Accepted Answer · 2011-04-29 19:06:22Z

12

Prelude> filter (/='!') "!!some!!_!!string!!"
"some_string"

The type of filter is (a -> Bool) -> [a] -> [a]. Since the 2nd argument is a String = [Char], meaning [a] = String = [Char], we infer that a must be a Char. The function therefore must take a Char as input. Therefore, you need to use '!', not "!".

answered Apr 29, 2011 at 19:06

kennytm

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3 Comments

user395760 Over a year ago

+1 Say what you will about Hindley-Milner-ish type systems, but they do make type signatures more useful.

kiltek Over a year ago

i'm not saying anything, i just want my code to work easily. And strong typing combined to functional programming has its pitfalls.

Carl Over a year ago

@kiltek Pitfalls like having to say what you mean? This isn't a nit-picky error. You were asking it to remove all elements of a list that matched something that couldn't possibly be an element of the list. That's a bug. The type system caught it at compile time. That seems like a pretty good deal to me.

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