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

31 Mar 2004


      Am Mittwoch, 31. März 2004 00:11 schrieb S. Alexander Jacobson:
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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'

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(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-
Wolfgang

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

Wolfgang Jeltsch