Question

I have the following function that iterates over a list [(Map String SqlValue)]

extractPatternStrings ∷ IO [(Map String SqlValue)] → IO [String] 
extractPatternStrings [] = do
    return []
extractPatternStrings lst = do
    (m:ms) ←  lst
    return $ (toString m) : (extractPatternStrings ms)
    where
        toString ∷  Map String SqlValue → String 
        toString m = (fromSql . fromJust . (Map.lookup "word"))∷ String

Execpt the empty list case is telling me that it couldn’t match the expected IO [Map String SqlValue] with actual [t0].

I thought the do = return would have taken care of this. How should I correct this?

Edit: To answer why I am using IO:

The function is being called from selectAll ↠ extractPatternStrings where selectAll reads from a database.

Answer 1

extractPatternStrings ∷ IO [(Map String SqlValue)] → IO [String]

An IO [String] is an IO action that produces a [String] result. Your use of do notation ensures extractPatternStrings produces an IO [String], not a [String].

An IO [(Map String SqlValue)] is an IO action that produces a [Map String SqlValue] result. But you cannot pattern match against an IO action. The syntax you use is for matching directly against a list, not against an IO action that produces a list.

You should use this type signature instead:

extractPatternStrings ∷ [Map String SqlValue] → IO [String] 

Except that, as @missingno points out, this doesn’t need to be an IO action:

extractPatternStrings ∷ [Map String SqlValue] → [String] 
extractPatternStrings []     = []
extractPatternStrings (m:ms) = toString m : extractPatternStrings ms
    where
        toString ∷  Map String SqlValue → String 
        toString m = (fromSql . fromJust . (Map.lookup "word"))∷ String

Or, better (and fixing an error in toString):

extractPatternStrings ∷ [Map String SqlValue] → [String] 
extractPatternStrings = map toString
    where
        toString ∷  Map String SqlValue → String 
        toString = fromSql . fromJust . Map.lookup "word"

More succinctly:

extractPatternStrings ∷ [Map String SqlValue] → [String] 
extractPatternStrings = map (fromSql . fromJust . Map.lookup "word")

If you really must have the original signature, then use liftM, either by changing your calling code to selectAll ↠ liftM extractPatternStrings (and I must confess I don’t recognise the operator you use there), or by defining extractPatternStrings as

extractPatternStrings ∷ IO [Map String SqlValue] → IO [String] 
extractPatternStrings = liftM $ map (fromSql . fromJust . Map.lookup "word")

But I recommend the former.