[Haskell-cafe] Re: Extending the idea of a general Num to other types?

Dan Piponi wrote:

On 9/5/07, Ketil Malde wrote:

...

On Wed, 2007-09-05 at 08:19 +0100, Simon Peyton-Jones wrote: Error message from GHCi: test/error.hs:2:8: No instance for (Num String) arising from use of `+' at test/error.hs:2:8-17 Possible fix: add an instance declaration for (Num String) In the expression: x + (show s) In the definition of `f': f x s = x + (show s)

...

your suggestion for the error message you'd like to have seen.

ghc --newbie-errors error.hs

. . . . . .

Error message from GHCi: test/error.hs:2:8: You have tried to apply the operator '+' to 'x' and 'show s' 'show s' is a String. I don't know how to apply '+' to a String. May I suggest either: (1) '+' is a method of type class Num. Tell me how to apply '+' to a String by making String an instance of the class Num (2) You didn't really mean '+' In the expression: x + (show s) In the definition of `f': f x s = x + (show s)

Splendid! And w/o the --newbie-errors drop the 'Possible fix:...'. In my experience, it is either unnecessary (because it is obvious which instance is missing) or (more often) misleading. As to the first line of the message 'No instance for (Num String)': I dislike the proposed 'String is not an instance of Num' for reasons already mentioned by others (multi parameter classes). I suggest to make it even shorter by directly quoting the missing syntax, i.e. 'Missing (instance Num String)'. Cheers Ben