#6005: Template Haskell disallows use of promoted data constructor in same
splice
---------------------------------------+------------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Component: Template Haskell
Version: 7.5 | Keywords: DataKinds PolyKinds
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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The following code fails to compile under GHC 7.5.20120413:
{{{
{-# LANGUAGE TemplateHaskell, DataKinds, PolyKinds #-}
$( [d|
data Nat = Zero | Succ Nat
data Proxy a = Proxy
foo :: Proxy Zero
foo = Proxy
|])
}}}
The error is:
{{{
The exact Name `Zero_a3N3' is not in scope
Probable cause: you used a unique name (NameU), perhaps via newName,
in Template Haskell, but did not bind it
If that's it, then -ddump-splices might be useful
}}}
Using {{{-ddump-splices}}} shows that the same internal name is used for
the data constructor at declaration and usage.
The problem seems to be that a data constructor declared within a splice
cannot be used as a promoted type in that same splice. Separating the
splices works.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6005>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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