help wanted with type error message

[Malcolm Wallace]

> In that case you need an existential type (not Haskell 98), which GHC
> and Hugs support thus:
> 
>	data Row a =  forall f. Formula f => Row (f a, Weight)

And incidentally, nhc98 also supports this particular extension with
this syntax.  (Hbc supports it too, but with a slightly different
syntax.)

Regards,
    Malcolm