On Jun 8, 2009, at 7:10 PM, Henry Laxen wrote:
convert :: (Data a, Data b) =Int -a -b
convert i x =
let c = dataTypeConstrs (dataTypeOf x) !! (i-1)
in fromConstr c
I would like to be able to say: x = convert 1 c and have it
assign Red to x then I would like to say: y = convert 1 s and
have it assign Small to y, however, when I try that I get:
Ambiguous type variable `b' in the constraint:
`Data b' arising from a use of `convert' at <interactive>:1:8-18
Probable fix: add a type signature that fixes these type
variable(s)
Of course if I say x :: Color = convert 1 c, it works, but I
would like to avoid that if possible, as all of the information
is already contained in the parameter c. Is there any way to do
this? Thanks in advance for your wise counsel.
Best wishes,
Henry Laxen
The type signature for 'convert' is throwing away the information you
want.
Try it with the following type signature and it should work fine:
convert :: (Data a) => Int -> a -> a
Of course, as has been noted, SYB is a rather big sledgehammer for
the insect in question.
Cheers,
S.
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