[Git][ghc/ghc][wip/issue-23821] Improvements to the documentation of defaulting plugins
Gergő Érdi (@cactus)
gitlab at gitlab.haskell.org
Thu Aug 31 06:47:57 UTC 2023
Gergő Érdi pushed to branch wip/issue-23821 at Glasgow Haskell Compiler / GHC
Commits:
b75f401b by Gergő Érdi at 2023-08-31T07:47:47+01:00
Improvements to the documentation of defaulting plugins
Based on @simonpj's draft and comments in !11117
- - - - -
4 changed files:
- compiler/GHC/Tc/Solver.hs
- compiler/GHC/Tc/Types.hs
- docs/users_guide/extending_ghc.rst
- testsuite/tests/plugins/defaulting-plugin/DefaultLifted.hs
Changes:
=====================================
compiler/GHC/Tc/Solver.hs
=====================================
@@ -3574,6 +3574,48 @@ beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs:
* Defaulting and disambiguation *
* *
*********************************************************************************
+
+Note [Defaulting plugins]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Defaulting plugins enable extending or overriding the defaulting
+behaviour. In `applyDefaulting`, before the built-in defaulting
+mechanism runs, the loaded defaulting plugins are passed the
+`WantedConstraints` and get a chance to propose defaulting assignments
+based on them.
+
+Proposals are represented as `[DefaultingProposal]` with each proposal
+consisting of a type variable to fill-in, the list of defaulting types to
+try in order, and a set of constraints to check at each try. This is
+the same representation (albeit in a nicely packaged-up data type) as
+the candidates generated by the built-in defaulting mechanism, so the
+actual trying of proposals is done by the same `disambigGroup` function.
+
+Wrinkle (DP1): The role of `WantedConstraints`
+
+ Plugins are passed `WantedConstraints` that can perhaps be
+ progressed on by defaulting. But a defaulting plugin is not a solver
+ plugin, its job is to provide defaulting proposals, i.e. mappings of
+ type variable to types. How do plugins know which type variables
+ they are supposed to default?
+
+ The `WantedConstraints` passed to the defaulting plugin are zonked
+ beforehand to ensure all remaining metavariables are unfilled. Thus,
+ the `WantedConstraints` serve a dual purpose: they are both the
+ constraints of the given context that can act as hints to the
+ defaulting, as well as the containers of the type variables under
+ consideration for defaulting.
+
+Wrinkle (DP2): Interactions between defaulting mechanisms
+
+ In the general case, we have multiple defaulting plugins loaded and
+ there is also the built-in defaulting mechanism. In this case, we
+ have to be careful to keep the `WantedConstraints` passed to the
+ plugins up-to-date by zonking between successful defaulting
+ rounds. Otherwise, two plugins might come up with a defaulting
+ proposal for the same metavariable; if the first one is accepted by
+ `disambigGroup` (thus the meta gets filled), the second proposal
+ becomes invalid (see #23821 for an example).
+
-}
applyDefaultingRules :: WantedConstraints -> TcS Bool
@@ -3590,6 +3632,8 @@ applyDefaultingRules wanteds
; tcg_env <- TcS.getGblEnv
; let plugins = tcg_defaulting_plugins tcg_env
+ -- Run any defaulting plugins
+ -- See Note [Defaulting plugins] for an overview
; (wanteds, plugin_defaulted) <- if null plugins then return (wanteds, []) else
do {
; traceTcS "defaultingPlugins {" (ppr wanteds)
@@ -3622,9 +3666,9 @@ applyDefaultingRules wanteds
[] -> return (wanteds, False)
_ -> do
-- If a defaulting plugin solves any tyvars, some of the wanteds
- -- will have filled-in metavars by now (see #23281). So we
- -- re-zonk to make sure later defaulting doesn't try to solve
- -- the same metavars.
+ -- will have filled-in metavars by now (see wrinkle DP2 of
+ -- Note [Defaulting plugins]). So we re-zonk to make sure later
+ -- defaulting doesn't try to solve the same metavars.
wanteds' <- TcS.zonkWC wanteds
return (wanteds', True)
}
=====================================
compiler/GHC/Tc/Types.hs
=====================================
@@ -86,7 +86,7 @@ module GHC.Tc.Types(
-- Defaulting plugin
DefaultingPlugin(..), DefaultingProposal(..),
- FillDefaulting, DefaultingPluginResult,
+ FillDefaulting,
-- Role annotations
RoleAnnotEnv, emptyRoleAnnotEnv, mkRoleAnnotEnv,
@@ -1069,8 +1069,12 @@ instance Outputable DefaultingProposal where
<+> ppr (deProposalCandidates p)
<+> ppr (deProposalCts p)
-type DefaultingPluginResult = [DefaultingProposal]
-type FillDefaulting = WantedConstraints -> TcPluginM DefaultingPluginResult
+type FillDefaulting
+ = WantedConstraints
+ -- Zonked constraints containing the unfilled metavariables that
+ -- can be defaulted. See wrinkle (DP1) of Note [Defaulting plugins]
+ -- in GHC.Tc.Solver
+ -> TcPluginM [DefaultingProposal]
-- | A plugin for controlling defaulting.
data DefaultingPlugin = forall s. DefaultingPlugin
=====================================
docs/users_guide/extending_ghc.rst
=====================================
@@ -1368,8 +1368,7 @@ Defaulting plugins have a single access point in the `GHC.Tc.Types` module
-- ^ The constraints against which defaults are checked.
}
- type DefaultingPluginResult = [DefaultingProposal]
- type FillDefaulting = WantedConstraints -> TcPluginM DefaultingPluginResult
+ type FillDefaulting = WantedConstraints -> TcPluginM [DefaultingProposal]
-- | A plugin for controlling defaulting.
data DefaultingPlugin = forall s. DefaultingPlugin
@@ -1381,18 +1380,36 @@ Defaulting plugins have a single access point in the `GHC.Tc.Types` module
-- ^ Clean up after the plugin, when exiting the type-checker.
}
-
-The plugin gets a combination of wanted constraints which can be most easily
-broken down into simple wanted constraints with ``approximateWC``. The result of
-running the plugin should be a ``DefaultingPluginResult``, a list of types that
-should be attempted for a given type variable that is ambiguous in a given
-context. GHC will check if one of the proposals is acceptable in the given
-context and then default to it. The most robust context to provide is the list
-of all wanted constraints that mention the variable you are defaulting. If you
-leave out a constraint, the default will be accepted, and then potentially
-result in a type checker error if it is incompatible with one of the constraints
-you left out. This can be a useful way of forcing a default and reporting errors
-to the user.
+The plugin has type ``WantedConstraints -> [DefaultingProposal]``.
+
+* It is given the currently unsolved constraints.
+* It returns a list of independent "defaulting proposals".
+* Each proposal of type ``DefaultingProposal`` specifies:
+ * ``deProposalTyVar``, ``deProposalCandidates``: specifies a list,
+ in priority order, of type to assign to that type variable
+ * ``deProposalCts :: [Ct]`` gives a set of constraints (always a
+ subset of the incoming ``WantedConstraints``) to use as a
+ criterion for acceptance
+
+After calling the plugin, GHC executes each ``DefaultingProposal`` in
+turn. To "execute" a proposal, GHC tries each of the proposed type
+assignments in ``deProposalCandidates`` in turn:
+
+* It assigns the proposed type to the type variable, and then tries to
+ solve ``deProposalCts``
+* If those constraints are completely solved by the assignment, GHC
+ accepts the assignment and moves on to the next ``DefaultingPropsal``
+* If not, GHC tries the next assignment in ``deProposalCandidates``.
+
+The plugin can assume that the incoming constraints are fully
+"zonked" (see :ghc-wiki:`the Wiki page on zonking <zonking>`).
+
+The most robust ``deProposalCts`` to provide is the list of all wanted
+constraints that mention the variable you are defaulting. If you leave
+out a constraint, the default may be accepted, and then potentially
+result in a type checker error if it is incompatible with one of the
+constraints you left out. This can be a useful way of forcing a
+default and reporting errors to the user.
There is an example of defaulting lifted types in the GHC test suite. In the
`testsuite/tests/plugins/` directory see `defaulting-plugin/` for the
=====================================
testsuite/tests/plugins/defaulting-plugin/DefaultLifted.hs
=====================================
@@ -68,7 +68,7 @@ data PluginState = PluginState { defaultClassName :: Name }
lookupName :: Module -> OccName -> TcPluginM Name
lookupName md occ = lookupOrig md occ
-solveDefaultType :: PluginState -> [Ct] -> TcPluginM DefaultingPluginResult
+solveDefaultType :: PluginState -> [Ct] -> TcPluginM [DefaultingProposal]
solveDefaultType _ [] = return []
solveDefaultType state wanteds = do
envs <- getInstEnvs
@@ -103,7 +103,7 @@ initialize :: TcPluginM PluginState
initialize = do
lookupDefaultTypes
-run :: PluginState -> WantedConstraints -> TcPluginM DefaultingPluginResult
+run :: PluginState -> WantedConstraints -> TcPluginM [DefaultingProposal]
run s ws = do
solveDefaultType s (ctsElts $ approximateWC False ws)
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b75f401b3760906cb1d85734fb35f14d942a750c
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b75f401b3760906cb1d85734fb35f14d942a750c
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