#6089: Allow declaration splices inside declaration brackets
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Reporter: igloo | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Template Haskell | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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I would like to be able to write code like:
{{{
{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
decs :: Q [Dec]
decs = [d| data MyType = MyCon | MyOtherCon
val1 :: MyType
val1 = MyCon
val2 :: MyType
val2 = MyOtherCon
$( do let mkFun v i = [| if $v == i then val1 else val2 |]
[d| fun3 x = $(mkFun [| x |] 3)
fun4 x = $(mkFun [| x |] 4) |] ) |]
}}}
but GHC says:
{{{
$ ghci decSplices.hs
GHCi, version 7.5.20120420: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( decSplices.hs, interpreted )
decSplices.hs:15:15:
Declaration splices are not permitted inside declaration brackets
Failed, modules loaded: none.
}}}
As far as I can see there is no technical reason why this couldn't work,
with a splice within a declaration bracket splitting the declarations into
3. The declarations before the splice would be renamed, typechecked etc
first, then those in the splice, and finally those after the splice.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6089>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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