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<h3>
Sylvain Henry pushed to branch wip/proposal-195
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/e36b0d435a9270c871e5180cf30909ddfad26940">e36b0d43</a></strong>
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<span>by Matthew Pickering</span>
<i>at 2020-07-09T15:43:54+02:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">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.
</pre>
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<a href="#06764eb0158306b83ab1998d18316392a51838c2">
compiler/GHC/Builtin/Names/TH.hs
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compiler/GHC/Tc/Deriv/Generate.hs
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compiler/GHC/Tc/Gen/Splice.hs
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docs/users_guide/exts/template_haskell.rst
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libraries/template-haskell/Language/Haskell/TH/CodeDo.hs
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libraries/template-haskell/Language/Haskell/TH/Lib.hs
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libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
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libraries/template-haskell/Language/Haskell/TH/Syntax.hs
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libraries/template-haskell/changelog.md
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libraries/text
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testsuite/tests/deriving/should_compile/drv-empty-data.stderr
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testsuite/tests/parser/should_compile/Proposal229f_instances.hs
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testsuite/tests/partial-sigs/should_compile/TypedSplice.hs
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testsuite/tests/quotes/T17857.hs
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testsuite/tests/th/TH_StringLift.hs
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testsuite/tests/th/TH_reifyLocalDefs.hs
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testsuite/tests/th/overloaded/T17839.hs
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testsuite/tests/th/overloaded/TH_overloaded_constraints.hs
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testsuite/tests/th/overloaded/TH_overloaded_csp.hs
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