#4125: Template haskell rejects duplicate instances before they're spliced
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Reporter: lilac | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Template Haskell | Version: 6.12.1
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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Comment(by simonpj):
I can see why you think it's ridiculous, but there an underlying reason.
Remember that TH quotations are ''typechecked''. So if you write `[| map
f [a,b] |]`, TH checks that the argument of `map` is compatible with
`map`'s type. To do that, it uses the type environment from outside the
quote, which binds `map`.
Your example contains an instance declaration. If you saw `[| return True
:: T Bool |]` you'd probably expect that to typecheck if (and only if) `T`
is an instance of `Monad`. Again, the instance environment comes from
outside the quote.
In this case it's a declaration splice but the story is the same: the
declarations are typechecked in the type/instance environment from outside
the quote; and that's why you get the complaint about a duplicate.
So I'm going to flag this as "invalid", not because it's silly but because
GHC is behaving according to specification at the moment. I suppose one
could imagine some kind of feature that would support what you want, but
I'm not sure what the design would be. If you feel like producing a
design, by all means open a new feature request.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4125#comment:1>
GHC <http://www.haskell.org/ghc/>
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