> I do not understand "not portable" here. Do you mean that some architectures
> don't support TH?
Template Haskell doesn't play nicely with cross compilation [1] or
stage-1 compilers, so Template Haskell is simply a non-starter for a
lot of uses. It's why the GHC codebase and boot libraries largely
don't use the -XTemplateHaskell extension (aside from the test suite,
of course).
> What staging issues?
>
> I'm imagining here having `deriving Blah` be surface syntax that desugars
> into some TH splice. You keep the nice user-facing syntax, but make the
> deriving mechanism itself specified in TH code.
That won't currently work with the way TH stages its splices. For
example, the following code:
{-# LANGUAGE PackageImports, TemplateHaskell #-}
import "deriving-compat" Data.Eq.Deriving
bar :: Bar
bar = Bar
data Foo = Foo
$(deriveEq ''Foo)
data Bar = Bar
$(deriveEq ''Bar)
will fail to compile because of the staging restrictions on Template
Haskell splices, whereas replacing the splices with `deriving Eq`
would make it compile.
> It's an interesting idea, but one probably best tackled after the current
> proposal.
Completely agreed. :)
Ryan S.
-----
[1] https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/CrossCompilation
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