[Haskell-cafe] Missing a "Deriving"?
ketil at malde.org
Sat May 30 17:37:57 EDT 2009
michael rice <nowgate at yahoo.com> writes:
> The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT.
> It compiles fine, but upon trying it I get the following error message.
> It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'.
My guess is that your expression is polymorphic, returning
Computation c => c [Int]
Since Haskell knows how to show this for both of the instances defined
for Computation (i.e. Maybe [Int] and [[Int]]), perhaps you could give
one of those as an explicit type signature?
If I haven't seen further, it is by standing in the footprints of giants
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