Gergely Risko gergely at
Wed Nov 6 15:08:53 UTC 2013

[sorry if someone receives this twice, apparently gmane has some issues with crossposting]
[this is also my reply to Ekmett's comment]

On Wed, 6 Nov 2013 12:06:04 +0000, Simon Peyton-Jones <simonpj at> writes:

> But this is true for ANY language extension. If a TH library exports a
> function that generates (say) a type family declaration, and you
> splice that into a file, do you need -XTypeFamilies in the client
> file?   I think currently you do.

So, we've made a little bit of testing on this.

There are a lot of extensions that simply can't be used with TH:
  - n+k,
  - RecursiveDo,
  - TransformListComp,
  - Arrows,
  - ImplicitParams,
  - TupleSections,
  - Monadcomprehensions.

The rest can be grouped into two parts.

The following extensions still work when spliced in without the
corresponding language pragma:
  - UnicodeSyntax,
  - LambdaCase,
  - NamedFieldPuns,
  - RecordWildCards,
  - DataTypeContexts (and you get rid of the deprecation warning
                      generation this way :)),
  - ConstraintKind,
  - MagicHash (note that UnboxedTuples is in the other part),
  - TraditionalRecordSyntax,
  - MultiWayIf,
  - GADTs (extra nice example at the end of this message).

The following needs the pragma at the place of splicing:
  - PostfixOperators,
  - ScopedTypeVariables,
  - Rank2, RankN,
  - deriving typeable and data,
  - UnboxedTuples,
  - ViewPatterns,
  - ParallelListComp,
  - ExistentialQuantification,
  - EmptyDataDecls,
  - TypeFamilies,
  - MultiParamTypeClasses,
  - FunctionalDependencies.

I don't see any trivial distinction, like based on Reader vs Typechecker
or anything like that.
Note ViewPatterns vs LambdaCase.
Note GADTs vs Rank2.

A very interesting example is ExplicitForAll.  The AST for polymorphic
functions always have explicit foralls in TH.Syntax; so there is no way
to require the user at the point of splicing to enable the language

GADTs are cool to:
{-# LANGUAGE TemplateHaskell #-}
-- No need for GADTs at all!
-- {-# LANGUAGE GADTs #-}

         data Foo where
           Foo1 :: Int -> Foo
           Foo2 :: String -> Foo

         f1 :: Foo
         f1 = Foo1 5

         f :: Foo -> Either Int String
         f (Foo1 n) = Left n
         f (Foo2 s) = Right s

main = print (f f1)

So all I'm asking for is that if it's not very inconvenient for the
implementor, please put the Annotations language pragma into the first
group. :)


More information about the ghc-devs mailing list