Haskell uppercase character map -- confusing error

Question

As some of you may know, the type inference errors in Haskell can be cryptic at times. I was trying to write a function that mapped alphabetic characters to their uppercase version, and I came up with this:

toUpper :: Char -> Char toUpper char = maybe " " (\a -> a) isValue where charMap = zip ['a' .. 'z'] ['A' .. 'Z'] isValue = lookup char charMap

But it complains about the following:

 wordsearch.hs:2:35: Couldn't match expected type `[Char]' against inferred type `Char' Expected type: Maybe [Char] Inferred type: Maybe Char In the third argument of `maybe', namely `isValue' In the expression: maybe " " (\ a -> a) isValue

The error is doesn't make sense to me as i'm a newbie, can anyone help?

ephemient · Accepted Answer · 2010-01-07 20:05:48Z

9

" " :: [Char] ' ' :: Char

You want maybe ' ' id isValue, not maybe " " id isValue.

answered Jan 7, 2010 at 20:05

ephemient

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1 Comment

RCIX Over a year ago

Ugh, i've got to pay more attention to these things. Thanks!

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