[Haskell-cafe] Template Haskell messes up scoping?

Stephan Friedrichs deduktionstheorem at web.de
Sun Mar 29 13:40:19 EDT 2009


when I tried to reorganize some code to use the data-accessor and
data-accessor-template packages, i stumbled across a strange effect:
When using template haskell some things are out of scope that really
shouldn't be. Let me give an example:

============== T.hs ==============
{-# LANGUAGE TemplateHaskell #-}

data Foo = Foo
    { bar :: Bar

$( return [] )

data Bar = Bar

> ghci T.hs
GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( T.hs, interpreted )

T.hs:4:13: Not in scope: type constructor or class `Bar'
Failed, modules loaded: none.

No matter what you do in $( ... ), the code doesn't compile. Even in the
simple case above, it doesn't work. What's happening here? Bug or
feature? And do you know a workaround?



Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr

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