[this is also my reply to Ekmett's comment]
On Wed, 6 Nov 2013 12:06:04 +0000, Simon Peyton-Jones <simo...@microsoft.com>
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
extension.
GADTs are cool to:
------------------------------
{-# LANGUAGE TemplateHaskell #-}
-- No need for GADTs at all!
-- {-# LANGUAGE GADTs #-}
$([d|
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. :)
Thanks,
Gergely
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