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|
{-# LANGUAGE RecordWildCards #-}
module NullaryPlugin where
-- base
import Data.Maybe
( catMaybes )
-- ghc
import GHC.Core.Class
( Class(..) )
import GHC.Core.DataCon
( classDataCon )
import GHC.Core.Make
( mkCoreConApps )
import GHC.Plugins
( Plugin )
import GHC.Tc.Plugin
( TcPluginM )
import GHC.Tc.Types
( TcPluginSolveResult(..) )
import GHC.Tc.Types.Constraint
( Ct(..) )
import GHC.Tc.Types.Evidence
( EvBindsVar, EvTerm(EvExpr) )
-- common
import Common
( PluginDefs(..)
, mkPlugin, don'tRewrite
)
--------------------------------------------------------------------------------
-- This plugin solves Wanted 'Nullary' constraints.
-- To do this, we just look through the Wanteds,
-- find any constraint whose className matches that of 'Nullary',
-- in which case we provide evidence (a nullary dictionary).
plugin :: Plugin
plugin = mkPlugin solver don'tRewrite
-- Solve "Nullary".
solver :: [String]
-> PluginDefs -> EvBindsVar -> [Ct] -> [Ct] -> [Ct]
-> TcPluginM TcPluginSolveResult
solver _args defs _ev _gs _ds ws = do
solved <- catMaybes <$> traverse ( solveCt defs ) ws
pure $ TcPluginOk solved []
solveCt :: PluginDefs -> Ct -> TcPluginM ( Maybe (EvTerm, Ct) )
solveCt ( PluginDefs {..} ) ct@( CDictCan { cc_class } )
| className cc_class == className nullary
, let
evTerm :: EvTerm
evTerm = EvExpr $ mkCoreConApps ( classDataCon cc_class ) []
= pure $ Just ( evTerm, ct )
solveCt _ ct = pure Nothing
|
