Various poor type error messages
I read the summary of the survey and noticed you wanted feedback on where error messages could be improved. I looked up some (simple) examples of type errors and ran them through ghc. I do not make any claims to be an HCI expert, just a mere mortal with an opinion.
Type error 1
module Test2 where fib n = if (3 > n) then 1 else (fib (n - 1) + fib (n - 2)) k = fib 's'
Test2.hs:4: No instance for (Num Char) arising from use of `fib' at Test2.hs:4 In the definition of `k': k = fib 's'
This isn't a bad error message in my humble opinion, it does pinpoint that I'm doing something wrong in line 4, and that there isn't an instance for Num Char doesn't come as a surprise. However I think it could have been more helpful by telling me that I tried to pass a Char to a function which expected an (Ord a, Num a) => a as its parameter.
Type error 2
module Test4 where k :: Int -> Int k l = 2.0*l
Test4.hs:4: No instance for (Fractional Int) arising from the literal `2.0' at Test4.hs:4 In the first argument of `(*)', namely `2.0' In the definition of `k': k l = 2.0 * l
One reason this kind of error could happen is an inexperienced user declaring the wrong type for his function, or not knowing that 2.0 would be interpreted as a Fractional.
Type error 3
module Test7 where len' xs = head (xs) + (length xs) o = len' "GH"
Test7.hs:4: Couldn't match `Int' against `Char' Expected type: [Int] Inferred type: [Char] In the first argument of `len'', namely `"GH"' In the definition of `o': o = len' "GH"
I ran this through Hugs version November 2002 and got this error message:
ERROR "Test7.hs":4 - Type error in application *** Expression : len' "GH" *** Term : "GH" *** Type : String *** Does not match : [Int]
I find the Hugs message more clear, but that might be my background.
Type error 4
module Test8 where f = head 3
Test8.hs:3: No instance for (Num [a]) arising from the literal `3' at Test8.hs:3 Possible cause: the monomorphism restriction applied to the following: f :: a (bound at Test8.hs:3) Probable fix: give these definition(s) an explicit type signature In the first argument of `head', namely `3' In the definition of `f': f = head 3
This one I find outright scary. For "wrong = div 3 8 + 1/2" it gives an error message that somewhat helps me guess the error, but the above doesn't even come close to helping me.