[Template-haskell] query functions

[Simon Peyton-Jones]

Hmm.  A full check, which involves type classes, requires the function
to consult the available instance declarations, so it'd have to be
monadic: Type -> Type -> Q Bool

Just getting types into the syntax tree at all, which Tim and I now plan
to do, is probably a higher priority at the moment; indeed, it may solve
the problem you originally wanted typeMatches for.

Simon

| -----Original Message-----
| From: template-haskell-bounces at haskell.org
[mailto:template-haskell-bounces at haskell.org] On
| Behalf Of Abraham Egnor
| Sent: 12 July 2004 18:58
| To: template-haskell at haskell.org
| Subject: [Template-haskell] query functions
| 
| One bit of functionality I've recently found a need for is various
| query functions in TH, i.e. something like
| 
| typeMatches :: Type -> Type -> Bool
| 
| that tests if the second type is a valid specialization of the first
| one.  The main need I've had for this is to test typeclass membership,
| but it would also be useful so that TH code can maintain a mapping of
| user-specified types to actions or suchlike.
| 
| Any chance of something like this getting into a future TH revision?
| 
| Abe
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