#3850: EmptyDataDecls and type context
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Reporter: Paczesiowa | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.1
Keywords: emptydatadecls | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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Comment(by guest):
I believe contexts on data declarations might not be totally useless, even
if there are no constructors. The HList code in question had the
following declaration
data HNat x => Label x ns desc -- labels are exclusively type-level
entities
The context HNat is asserted not because we want to use any of HNat's
methods. Rather, the constraint HNat was asserted as a kind predicate --
to
show to the user that the type variable x is of a kind natural
numbers, so to speak. Therefore, the type (Label HZero Foo Foo) is the
correct instance of label. On the other hand, the type (Label Bool
Foo Foo) is not a correct instance and will be flagged as such by the
compiler.
Oleg
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3850#comment:3>
GHC <http://www.haskell.org/ghc/>
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