Ralf Lammel wrote:
As Bulat points out, the GHC primitive dataToTag#
indeed nicely solves the problem. Ben, just for
completeness' sake; with SYB, you get such reflective
information too (and others):
shallowEq :: Data a => a -> a -> Bool
shallowEq x y = toConstr x == toConstr y
(dataToTag# returns Int, while toConstr comprises other things like the
constructor name.)
Ralf,
Yes, I ended up using the "propper" SYB approach instead, though I have
noticed that the reflection data types Constr and DataRep make no
mention of type variables or functions.
For example, this works fine:
> getTag (Just 5) ==# getTag (Just{})
> getTag (Just (\x -> x)) ==# getTag (Just{})
But this does not
> toConstr (Just 5) == toConstr (Just{})
Ambiguous type variables.
> toConstr (Just (\x -> x)) == toConstr (Just{})
No instance for Data (t -> t)
I appreciate the reasons why this is so, though I think it's interesting
to see the practical consequences.
A toConstr version of shallowEq works ok so long as you provide a type
signature to constrain both arguments to be the same type, and one of
them is always fully constructed - which is fine for me at the moment.
Ben.
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