[Git][ghc/ghc][wip/proposal-195] Use a newtype `Code` for the return type of typed quotations (Proposal #195)

[Sylvain Henry]

Sylvain Henry pushed to branch wip/proposal-195 at Glasgow Haskell Compiler / GHC


Commits:
f07a8c8c by Matthew Pickering at 2020-07-20T10:20:01+02:00
Use a newtype `Code` for the return type of typed quotations (Proposal #195)

There are three problems with the current API:

1. It is hard to properly write instances for ``Quote m => m (TExp a)`` as the type is the composition
   of two type constructors. Doing so in your program involves making your own newtype and
   doing a lot of wrapping/unwrapping.

   For example, if I want to create a language which I can either run immediately or
   generate code from I could write the following with the new API. ::

      class Lang r where
        _int :: Int -> r Int
        _if  :: r Bool -> r a -> r a -> r a

      instance Lang Identity where
        _int = Identity
        _if (Identity b) (Identity t) (Identity f) = Identity (if b then t else f)

      instance Quote m => Lang (Code m) where
        _int = liftTyped
        _if cb ct cf = [|| if $$cb then $$ct else $$cf ||]

2. When doing code generation it is common to want to store code fragments in
   a map. When doing typed code generation, these code fragments contain a
   type index so it is desirable to store them in one of the parameterised
   map data types such as ``DMap`` from ``dependent-map`` or ``MapF`` from
   ``parameterized-utils``.

   ::

      compiler :: Env -> AST a -> Code Q a

      data AST a where ...
      data Ident a = ...

      type Env = MapF Ident (Code Q)

      newtype Code m a = Code (m (TExp a))

   In this example, the ``MapF`` maps an ``Ident String`` directly to a ``Code Q String``.
   Using one of these map types currently requires creating your own newtype and constantly
   wrapping every quotation and unwrapping it when using a splice. Achievable, but
   it creates even more syntactic noise than normal metaprogramming.

3. ``m (TExp a)`` is ugly to read and write, understanding ``Code m a`` is
   easier. This is a weak reason but one everyone
   can surely agree with.

Updates text submodule.

- - - - -


30 changed files:

- compiler/GHC/Builtin/Names/TH.hs
- compiler/GHC/Tc/Deriv/Generate.hs
- compiler/GHC/Tc/Gen/Splice.hs
- docs/users_guide/exts/deriving_extra.rst
- docs/users_guide/exts/template_haskell.rst
- libraries/template-haskell/Language/Haskell/TH.hs
- + libraries/template-haskell/Language/Haskell/TH/CodeDo.hs
- libraries/template-haskell/Language/Haskell/TH/Lib.hs
- libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
- libraries/template-haskell/Language/Haskell/TH/Syntax.hs
- libraries/template-haskell/changelog.md
- libraries/template-haskell/template-haskell.cabal.in
- libraries/text
- testsuite/tests/deriving/should_compile/drv-empty-data.stderr
- testsuite/tests/parser/should_compile/Proposal229f_instances.hs
- testsuite/tests/partial-sigs/should_compile/TypedSplice.hs
- testsuite/tests/partial-sigs/should_compile/TypedSplice.stderr
- testsuite/tests/quotes/T17857.hs
- testsuite/tests/th/T10945.stderr
- testsuite/tests/th/T11452.stderr
- testsuite/tests/th/T15471A.hs
- testsuite/tests/th/T15843.hs
- testsuite/tests/th/T16195A.hs
- testsuite/tests/th/T18121.hs
- testsuite/tests/th/T8577.stderr
- testsuite/tests/th/T8577a.hs
- testsuite/tests/th/TH_StringLift.hs
- testsuite/tests/th/TH_reifyLocalDefs.hs
- testsuite/tests/th/overloaded/T17839.hs
- testsuite/tests/th/overloaded/TH_overloaded_constraints.hs


The diff was not included because it is too large.


View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/f07a8c8c462b9a7cf2221223c543055193d8a653

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/f07a8c8c462b9a7cf2221223c543055193d8a653
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