CODEWITHSUNDEEP
haskellfunctional-programming
SirTee
SirTee

Reputation: 23

Haskell couldn't match expected type [char] with actual type IO

I just started learning Haskell, and i'm on stuck on this error among a bunch of others

i'm trying to print all the characters in the char list recursively with this code 

printall :: [Char] -> [Char]
printall "" = ""
printall (i:is) = if is /= "" then print i else printall is

main = printall "hello world"

but i get this error could anyone help me?

intro.hs:14:36: error:
• Couldn't match expected type ‘[Char]’ with actual type
‘IO ()’
• In the expression: print i
  In the expression: if is /= "" then print i else printa
ll is
  In an equation for ‘printall’:
      printall (i : is) = if is /= "" then print i else p
 rintall is

intro.hs:16:1: error:
• Couldn't match expected type ‘IO t0’ with actual type ‘
[Char]’

Upvotes: 2

Views: 2997

Answers (2)

Euge
Euge

Reputation: 749

As you said in a comment above, each branch of the if clause should have the same type, indeed.

Also, main function must always have IO a type, for some a, which is usually (). This means that the type signature for printall should be:

printall :: [Char] -> IO ()

which is the same as:

printall :: String -> IO ()

Upvotes: 3

wizzup
wizzup

Reputation: 2411

print type is print :: Show a => a -> IO ()

but your printall type is printall :: [Char] -> [Char], assume it is what you want.

The expression if x then y else z need the same type for y and z

Upvotes: 0

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