[Haskell-cafe] Adding Ord constraint to instance Monad Set?

Wolfgang Jeltsch wolfgang at jeltsch.net
Wed Mar 31 08:44:54 EST 2004

Am Mittwoch, 31. März 2004 00:11 schrieb S. Alexander Jacobson:
> [...]

>     Could not deduce (Ord b) from the context (Monad Set)
>       arising from use of `concatSets' at dbMeta3.hs:242
>     Probable fix:
>         Add (Ord b) to the class or instance method `>>='
>     In the definition of `>>=': >>= m k = concatSets (mapSet k m)
>     In the definition for method `>>='
>     In the instance declaration for `Monad Set'

> [...]

> (Aside: it be really nice if the error messages
> suggested textual changes or at least provide
> sample syntax in addition to the conceptual
> recommendation.)

Hey, what do you want from a compiler?  That it writes you your code? ;-)  
IMO, error messages like the one above are *very* useful; they give you hints 
about what you can do.  If you don't know what the hints mean, you have to 
have a look at the Report or whatever.  (Well, in this special case, I have 
to admit that I also don't know what is meant with "adding a constraint to a 
class or instance method".  I'd say: "adding a constraint to a class, a 
method (declared with the class) or an instance".)

> -Alex-


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