#589: Various poor type error messages
-----------------------------------------------+----------------------------
Reporter: Peter A Jonsson [[email protected]] | Owner:
Type: bug | Status: new
Priority: low | Milestone: _|_
Component: Compiler (Type checker) | Version: 6.4.1
Resolution: | Keywords:
Difficulty: Unknown | Os:
Unknown/Multiple
Testcase: | Architecture:
Unknown/Multiple
Failure: None/Unknown |
-----------------------------------------------+----------------------------
Changes (by igloo):
* failure: => None/Unknown
Old description:
> {{{
> Hello,
>
> 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.
>
> Code:
>
> 1 module Test2 where
> 2
> 3 fib n = if (3 > n) then 1 else (fib (n - 1) + fib (n - 2))
> 4 k = fib 's'
>
> Error message:
>
> 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.
>
> Code:
>
> 1 module Test4 where
> 2
> 3 k :: Int -> Int
> 4 k l = 2.0*l
>
> Error message:
>
> 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.
>
> Code:
>
> 1 module Test7 where
> 2
> 3 len' xs = head (xs) + (length xs)
> 4 o = len' "GH"
>
> Error message:
>
> 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.
>
> Code:
>
> 1 module Test8 where
> 2
> 3 f = head 3
>
> Error message:
>
> 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.
>
> / Peter
> }}}
New description:
Hello,
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'''
Code:
{{{
1 module Test2 where
2
3 fib n = if (3 > n) then 1 else (fib (n - 1) + fib (n - 2))
4 k = fib 's'
}}}
Error message:
{{{
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'''
Code:
{{{
1 module Test4 where
2
3 k :: Int -> Int
4 k l = 2.0*l
}}}
Error message:
{{{
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'''
Code:
{{{
1 module Test7 where
2
3 len' xs = head (xs) + (length xs)
4 o = len' "GH"
}}}
Error message:
{{{
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'''
Code:
{{{
1 module Test8 where
2
3 f = head 3
}}}
Error message:
{{{
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.
/ Peter
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/589#comment:8>
GHC <http://www.haskell.org/ghc/>
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