X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcRnTypes.lhs;h=4abb40836f0f936b6b6a037d789e1757e32f5225;hp=feaf9f949200cfc9799bb5cfb764e5354bf70632;hb=b10d7d079ec9c3fc22d4700fe484dd297bddb805;hpb=aafdba3bce91afb003f5f50e001e141744837bae diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs index feaf9f9..4abb408 100644 --- a/compiler/typecheck/TcRnTypes.lhs +++ b/compiler/typecheck/TcRnTypes.lhs @@ -3,13 +3,6 @@ % (c) The GRASP Project, Glasgow University, 1992-2002 % \begin{code} -{-# OPTIONS -w #-} --- The above warning supression flag is a temporary kludge. --- While working on this module you are encouraged to remove it and fix --- any warnings in the module. See --- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings --- for details - module TcRnTypes( TcRnIf, TcRn, TcM, RnM, IfM, IfL, IfG, -- The monad is opaque outside this module TcRef, @@ -20,40 +13,51 @@ module TcRnTypes( IfGblEnv(..), IfLclEnv(..), -- Ranamer types - ErrCtxt, RecFieldEnv, + ErrCtxt, RecFieldEnv(..), ImportAvails(..), emptyImportAvails, plusImportAvails, WhereFrom(..), mkModDeps, -- Typechecker types - TcTyThing(..), pprTcTyThingCategory, RefinementVisibility(..), + TcTypeEnv, TcTyThing(..), pprTcTyThingCategory, -- Template Haskell - ThStage(..), topStage, topSpliceStage, - ThLevel, impLevel, topLevel, + ThStage(..), topStage, topAnnStage, topSpliceStage, + ThLevel, impLevel, outerLevel, thLevel, -- Arrows ArrowCtxt(NoArrowCtxt), newArrowScope, escapeArrowScope, - -- Insts - Inst(..), InstOrigin(..), InstLoc(..), - pprInstLoc, pprInstArising, instLocSpan, instLocOrigin, - LIE, emptyLIE, unitLIE, plusLIE, consLIE, instLoc, instSpan, - plusLIEs, mkLIE, isEmptyLIE, lieToList, listToLIE, + -- Constraints + Untouchables(..), inTouchableRange, isNoUntouchables, + WantedConstraints, emptyWanteds, andWanteds, extendWanteds, + WantedConstraint(..), WantedEvVar(..), wantedEvVarLoc, + wantedEvVarToVar, wantedEvVarPred, splitWanteds, + + evVarsToWanteds, + Implication(..), + CtLoc(..), ctLocSpan, ctLocOrigin, setCtLocOrigin, + CtOrigin(..), EqOrigin(..), + WantedLoc, GivenLoc, + + SkolemInfo(..), + + -- Pretty printing + pprEvVarTheta, pprWantedsWithLocs, pprWantedWithLoc, + pprEvVars, pprEvVarWithType, + pprArising, pprArisingAt, -- Misc other types - TcId, TcIdSet, TcDictBinds, + TcId, TcIdSet, TcTyVarBind(..), TcTyVarBinds ) where #include "HsVersions.h" -import HsSyn hiding (LIE) +import HsSyn import HscTypes -import Packages import Type -import Coercion import TcType -import TcGadt +import Annotations import InstEnv import FamInstEnv import IOEnv @@ -64,20 +68,20 @@ import NameSet import Var import VarEnv import Module -import UniqFM import SrcLoc import VarSet import ErrUtils +import UniqFM import UniqSupply +import Unique import BasicTypes -import Util import Bag import Outputable import ListSetOps -import FiniteMap +import FastString +import StaticFlags( opt_ErrorSpans ) -import Data.Maybe -import Data.List +import Data.Set (Set) \end{code} @@ -92,9 +96,9 @@ The monad itself has to be defined here, because it is mentioned by ErrCtxt \begin{code} type TcRef a = IORef a -type TcId = Id -- Type may be a TcType +type TcId = Id -- Type may be a TcType DV: WHAT?????????? type TcIdSet = IdSet -type TcDictBinds = DictBinds TcId -- Bag of dictionary bindings + type TcRnIf a b c = IOEnv (Env a b) c type IfM lcl a = TcRnIf IfGblEnv lcl a -- Iface stuff @@ -106,6 +110,18 @@ type RnM a = TcRn a -- Historical type TcM a = TcRn a -- Historical \end{code} +Representation of type bindings to uninstantiated meta variables used during +constraint solving. + +\begin{code} +data TcTyVarBind = TcTyVarBind TcTyVar TcType + +type TcTyVarBinds = Bag TcTyVarBind + +instance Outputable TcTyVarBind where + ppr (TcTyVarBind tv ty) = ppr tv <+> text ":=" <+> ppr ty +\end{code} + %************************************************************************ %* * @@ -134,23 +150,24 @@ data Env gbl lcl -- Changes as we move into an expression data TcGblEnv = TcGblEnv { - tcg_mod :: Module, -- Module being compiled - tcg_src :: HscSource, -- What kind of module - -- (regular Haskell, hs-boot, ext-core) + tcg_mod :: Module, -- ^ Module being compiled + tcg_src :: HscSource, + -- ^ What kind of module (regular Haskell, hs-boot, ext-core) - tcg_rdr_env :: GlobalRdrEnv, -- Top level envt; used during renaming - tcg_default :: Maybe [Type], -- Types used for defaulting - -- Nothing => no 'default' decl + tcg_rdr_env :: GlobalRdrEnv, -- ^ Top level envt; used during renaming + tcg_default :: Maybe [Type], + -- ^ Types used for defaulting. @Nothing@ => no @default@ decl - tcg_fix_env :: FixityEnv, -- Just for things in this module - tcg_field_env :: RecFieldEnv, -- Just for things in this module + tcg_fix_env :: FixityEnv, -- ^ Just for things in this module + tcg_field_env :: RecFieldEnv, -- ^ Just for things in this module - tcg_type_env :: TypeEnv, -- Global type env for the module we are compiling now - -- All TyCons and Classes (for this module) end up in here right away, - -- along with their derived constructors, selectors. - -- - -- (Ids defined in this module start in the local envt, - -- though they move to the global envt during zonking) + tcg_type_env :: TypeEnv, + -- ^ Global type env for the module we are compiling now. All + -- TyCons and Classes (for this module) end up in here right away, + -- along with their derived constructors, selectors. + -- + -- (Ids defined in this module start in the local envt, though they + -- move to the global envt during zonking) tcg_type_env_var :: TcRef TypeEnv, -- Used only to initialise the interface-file @@ -158,94 +175,118 @@ data TcGblEnv -- bound in this module when dealing with hi-boot recursions -- Updated at intervals (e.g. after dealing with types and classes) - tcg_inst_env :: InstEnv, -- Instance envt for *home-package* - -- modules; Includes the dfuns in - -- tcg_insts - tcg_fam_inst_env :: FamInstEnv, -- Ditto for family instances + tcg_inst_env :: InstEnv, + -- ^ Instance envt for /home-package/ modules; Includes the dfuns in + -- tcg_insts + tcg_fam_inst_env :: FamInstEnv, -- ^ Ditto for family instances -- Now a bunch of things about this module that are simply -- accumulated, but never consulted until the end. -- Nevertheless, it's convenient to accumulate them along -- with the rest of the info from this module. - tcg_exports :: [AvailInfo], -- What is exported - tcg_imports :: ImportAvails, -- Information about what was imported - -- from where, including things bound - -- in this module - - tcg_dus :: DefUses, -- What is defined in this module and what is used. - -- The latter is used to generate - -- (a) version tracking; no need to recompile if these - -- things have not changed version stamp - -- (b) unused-import info - - tcg_keep :: TcRef NameSet, -- Locally-defined top-level names to keep alive - -- "Keep alive" means give them an Exported flag, so - -- that the simplifier does not discard them as dead - -- code, and so that they are exposed in the interface file - -- (but not to export to the user). - -- - -- Some things, like dict-fun Ids and default-method Ids are - -- "born" with the Exported flag on, for exactly the above reason, - -- but some we only discover as we go. Specifically: - -- * The to/from functions for generic data types - -- * Top-level variables appearing free in the RHS of an orphan rule - -- * Top-level variables appearing free in a TH bracket - - tcg_inst_uses :: TcRef NameSet, -- Home-package Dfuns actually used - -- Used to generate version dependencies - -- This records usages, rather like tcg_dus, but it has to - -- be a mutable variable so it can be augmented - -- when we look up an instance. These uses of dfuns are - -- rather like the free variables of the program, but - -- are implicit instead of explicit. - - tcg_th_used :: TcRef Bool, -- True <=> Template Haskell syntax used - -- We need this so that we can generate a dependency on the - -- Template Haskell package, becuase the desugarer is going to - -- emit loads of references to TH symbols. It's rather like - -- tcg_inst_uses; the reference is implicit rather than explicit, - -- so we have to zap a mutable variable. - - tcg_dfun_n :: TcRef Int, -- Allows us to number off the names of DFuns - -- It's convenient to allocate an External Name for a DFun, with - -- a permanently-fixed unique, just like other top-level functions - -- defined in this module. But that means we need a canonical - -- occurrence name, distinct from all other dfuns in this module, - -- and this name supply serves that purpose (df1, df2, etc). - - -- The next fields accumulate the payload of the module - -- The binds, rules and foreign-decl fiels are collected - -- initially in un-zonked form and are finally zonked in tcRnSrcDecls - - -- The next fields accumulate the payload of the - -- module The binds, rules and foreign-decl fiels are - -- collected initially in un-zonked form and are - -- finally zonked in tcRnSrcDecls - - tcg_rn_imports :: Maybe [LImportDecl Name], + tcg_exports :: [AvailInfo], -- ^ What is exported + tcg_imports :: ImportAvails, + -- ^ Information about what was imported from where, including + -- things bound in this module. + + tcg_dus :: DefUses, + -- ^ What is defined in this module and what is used. + -- The latter is used to generate + -- + -- (a) version tracking; no need to recompile if these things have + -- not changed version stamp + -- + -- (b) unused-import info + + tcg_keep :: TcRef NameSet, + -- ^ Locally-defined top-level names to keep alive. + -- + -- "Keep alive" means give them an Exported flag, so that the + -- simplifier does not discard them as dead code, and so that they + -- are exposed in the interface file (but not to export to the + -- user). + -- + -- Some things, like dict-fun Ids and default-method Ids are "born" + -- with the Exported flag on, for exactly the above reason, but some + -- we only discover as we go. Specifically: + -- + -- * The to/from functions for generic data types + -- + -- * Top-level variables appearing free in the RHS of an orphan + -- rule + -- + -- * Top-level variables appearing free in a TH bracket + + tcg_inst_uses :: TcRef NameSet, + -- ^ Home-package Dfuns actually used. + -- + -- Used to generate version dependencies This records usages, rather + -- like tcg_dus, but it has to be a mutable variable so it can be + -- augmented when we look up an instance. These uses of dfuns are + -- rather like the free variables of the program, but are implicit + -- instead of explicit. + + tcg_th_used :: TcRef Bool, + -- ^ @True@ <=> Template Haskell syntax used. + -- + -- We need this so that we can generate a dependency on the Template + -- Haskell package, becuase the desugarer is going to emit loads of + -- references to TH symbols. It's rather like tcg_inst_uses; the + -- reference is implicit rather than explicit, so we have to zap a + -- mutable variable. + + tcg_dfun_n :: TcRef OccSet, + -- ^ Allows us to choose unique DFun names. + + -- The next fields accumulate the payload of the module + -- The binds, rules and foreign-decl fiels are collected + -- initially in un-zonked form and are finally zonked in tcRnSrcDecls + tcg_rn_exports :: Maybe [Located (IE Name)], - tcg_rn_decls :: Maybe (HsGroup Name), -- renamed decls, maybe - -- Nothing <=> Don't retain renamed decls + tcg_rn_imports :: [LImportDecl Name], + -- Keep the renamed imports regardless. They are not + -- voluminous and are needed if you want to report unused imports + tcg_used_rdrnames :: TcRef (Set RdrName), + -- The set of used *imported* (not locally-defined) RdrNames + -- Used only to report unused import declarations + + tcg_rn_decls :: Maybe (HsGroup Name), + -- ^ Renamed decls, maybe. @Nothing@ <=> Don't retain renamed + -- decls. + + tcg_ev_binds :: Bag EvBind, -- Top-level evidence bindings tcg_binds :: LHsBinds Id, -- Value bindings in this module - tcg_deprecs :: Deprecations, -- ...Deprecations + tcg_sigs :: NameSet, -- ...Top-level names that *lack* a signature + tcg_imp_specs :: [LTcSpecPrag], -- ...SPECIALISE prags for imported Ids + tcg_warns :: Warnings, -- ...Warnings and deprecations + tcg_anns :: [Annotation], -- ...Annotations tcg_insts :: [Instance], -- ...Instances tcg_fam_insts :: [FamInst], -- ...Family instances tcg_rules :: [LRuleDecl Id], -- ...Rules tcg_fords :: [LForeignDecl Id], -- ...Foreign import & exports - tcg_doc :: Maybe (HsDoc Name), -- Maybe Haddock documentation - tcg_hmi :: HaddockModInfo Name, -- Haddock module information - tcg_hpc :: AnyHpcUsage -- True if any part of the prog uses hpc instrumentation. + tcg_doc_hdr :: Maybe LHsDocString, -- ^ Maybe Haddock header docs + tcg_hpc :: AnyHpcUsage, -- ^ @True@ if any part of the + -- prog uses hpc instrumentation. + + tcg_main :: Maybe Name -- ^ The Name of the main + -- function, if this module is + -- the main module. } -type RecFieldEnv = NameEnv [Name] -- Maps a constructor name *in this module* - -- to the fields for that constructor +data RecFieldEnv + = RecFields (NameEnv [Name]) -- Maps a constructor name *in this module* + -- to the fields for that constructor + NameSet -- Set of all fields declared *in this module*; + -- used to suppress name-shadowing complaints + -- when using record wild cards + -- E.g. let fld = e in C {..} -- This is used when dealing with ".." notation in record -- construction and pattern matching. - -- The FieldEnv deals *only* with constructors defined in - -- *thie* module. For imported modules, we get the same info - -- from the TypeEnv + -- The FieldEnv deals *only* with constructors defined in *this* + -- module. For imported modules, we get the same info from the + -- TypeEnv \end{code} %************************************************************************ @@ -315,11 +356,11 @@ data TcLclEnv -- Changes as we move inside an expression -- Discarded after typecheck/rename; not passed on to desugarer = TcLclEnv { tcl_loc :: SrcSpan, -- Source span - tcl_ctxt :: ErrCtxt, -- Error context + tcl_ctxt :: [ErrCtxt], -- Error context, innermost on top tcl_errs :: TcRef Messages, -- Place to accumulate errors - tcl_th_ctxt :: ThStage, -- Template Haskell context - tcl_arrow_ctxt :: ArrowCtxt, -- Arrow-notation context + tcl_th_ctxt :: ThStage, -- Template Haskell context + tcl_arrow_ctxt :: ArrowCtxt, -- Arrow-notation context tcl_rdr :: LocalRdrEnv, -- Local name envt -- Maintained during renaming, of course, but also during @@ -333,17 +374,27 @@ data TcLclEnv -- Changes as we move inside an expression -- We still need the unsullied global name env so that -- we can look up record field names - tcl_env :: NameEnv TcTyThing, -- The local type environment: Ids and TyVars - -- defined in this module + tcl_env :: TcTypeEnv, -- The local type environment: Ids and + -- TyVars defined in this module tcl_tyvars :: TcRef TcTyVarSet, -- The "global tyvars" -- Namely, the in-scope TyVars bound in tcl_env, - -- plus the tyvars mentioned in the types of Ids bound in tcl_lenv - -- Why mutable? see notes with tcGetGlobalTyVars - - tcl_lie :: TcRef LIE -- Place to accumulate type constraints + -- plus the tyvars mentioned in the types of Ids bound + -- in tcl_lenv. + -- Why mutable? see notes with tcGetGlobalTyVars + + tcl_lie :: TcRef WantedConstraints, -- Place to accumulate type constraints + + -- TcMetaTyVars have + tcl_meta :: TcRef Unique, -- The next free unique for TcMetaTyVars + -- Guaranteed to be allocated linearly + tcl_untch :: Unique -- Any TcMetaTyVar with + -- unique >= tcl_untch is touchable + -- unique < tcl_untch is untouchable } +type TcTypeEnv = NameEnv TcTyThing + {- Note [Given Insts] ~~~~~~~~~~~~~~~~~~ @@ -360,36 +411,55 @@ pass it inwards. -} --------------------------- --- Template Haskell levels +-- Template Haskell stages and levels --------------------------- +data ThStage -- See Note [Template Haskell state diagram] in TcSplice + = Splice -- Top-level splicing + -- This code will be run *at compile time*; + -- the result replaces the splice + -- Binding level = 0 + + | Comp -- Ordinary Haskell code + -- Binding level = 1 + + | Brack -- Inside brackets + ThStage -- Binding level = level(stage) + 1 + (TcRef [PendingSplice]) -- Accumulate pending splices here + (TcRef WantedConstraints) -- and type constraints here + +topStage, topAnnStage, topSpliceStage :: ThStage +topStage = Comp +topAnnStage = Splice +topSpliceStage = Splice + +instance Outputable ThStage where + ppr Splice = text "Splice" + ppr Comp = text "Comp" + ppr (Brack s _ _) = text "Brack" <> parens (ppr s) + type ThLevel = Int - -- Indicates how many levels of brackets we are inside - -- (always >= 0) + -- See Note [Template Haskell levels] in TcSplice -- Incremented when going inside a bracket, -- decremented when going inside a splice -- NB: ThLevel is one greater than the 'n' in Fig 2 of the -- original "Template meta-programming for Haskell" paper -impLevel, topLevel :: ThLevel -topLevel = 1 -- Things defined at top level of this module +impLevel, outerLevel :: ThLevel impLevel = 0 -- Imported things; they can be used inside a top level splice +outerLevel = 1 -- Things defined outside brackets +-- NB: Things at level 0 are not *necessarily* imported. +-- eg $( \b -> ... ) here b is bound at level 0 -- -- For example: -- f = ... -- g1 = $(map ...) is OK -- g2 = $(f ...) is not OK; because we havn't compiled f yet - -data ThStage - = Comp -- Ordinary compiling, at level topLevel - | Splice ThLevel -- Inside a splice - | Brack ThLevel -- Inside brackets; - (TcRef [PendingSplice]) -- accumulate pending splices here - (TcRef LIE) -- and type constraints here -topStage, topSpliceStage :: ThStage -topStage = Comp -topSpliceStage = Splice (topLevel - 1) -- Stage for the body of a top-level splice +thLevel :: ThStage -> ThLevel +thLevel Splice = 0 +thLevel Comp = 1 +thLevel (Brack s _ _) = thLevel s + 1 --------------------------- -- Arrow-notation context @@ -439,63 +509,41 @@ data TcTyThing = AGlobal TyThing -- Used only in the return type of a lookup | ATcId { -- Ids defined in this module; may not be fully zonked - tct_id :: TcId, - tct_co :: RefinementVisibility, -- Previously: Maybe HsWrapper - -- Nothing <=> Do not apply a GADT type refinement - -- I am wobbly, or have no free - -- type variables - -- Just co <=> Apply any type refinement to me, - -- and record it in the coercion - tct_type :: TcType, -- Type of (coercion applied to id) + tct_id :: TcId, tct_level :: ThLevel } | ATyVar Name TcType -- The type to which the lexically scoped type vaiable -- is currently refined. We only need the Name - -- for error-message purposes + -- for error-message purposes; it is the corresponding + -- Name in the domain of the envt | AThing TcKind -- Used temporarily, during kind checking, for the -- tycons and clases in this recursive group -data RefinementVisibility - = Unrefineable -- Do not apply a GADT refinement - -- I have no free variables - - | Rigid HsWrapper -- Apply any refinement to me - -- and record it in the coercion - - | Wobbly -- Do not apply a GADT refinement - -- I am wobbly - - | WobblyInvisible -- Wobbly type, not available inside current - -- GADT refinement - instance Outputable TcTyThing where -- Debugging only ppr (AGlobal g) = pprTyThing g ppr elt@(ATcId {}) = text "Identifier" <> - ifPprDebug (brackets (ppr (tct_id elt) <> dcolon <> ppr (tct_type elt) <> comma - <+> ppr (tct_level elt) <+> ppr (tct_co elt))) + brackets (ppr (tct_id elt) <> dcolon + <> ppr (varType (tct_id elt)) <> comma + <+> ppr (tct_level elt)) ppr (ATyVar tv _) = text "Type variable" <+> quotes (ppr tv) ppr (AThing k) = text "AThing" <+> ppr k pprTcTyThingCategory :: TcTyThing -> SDoc pprTcTyThingCategory (AGlobal thing) = pprTyThingCategory thing -pprTcTyThingCategory (ATyVar {}) = ptext SLIT("Type variable") -pprTcTyThingCategory (ATcId {}) = ptext SLIT("Local identifier") -pprTcTyThingCategory (AThing {}) = ptext SLIT("Kinded thing") - -instance Outputable RefinementVisibility where - ppr Unrefineable = ptext SLIT("unrefineable") - ppr (Rigid co) = ptext SLIT("rigid") <+> ppr co - ppr Wobbly = ptext SLIT("wobbly") - ppr WobblyInvisible = ptext SLIT("wobbly-invisible") - +pprTcTyThingCategory (ATyVar {}) = ptext (sLit "Type variable") +pprTcTyThingCategory (ATcId {}) = ptext (sLit "Local identifier") +pprTcTyThingCategory (AThing {}) = ptext (sLit "Kinded thing") \end{code} \begin{code} -type ErrCtxt = [TidyEnv -> TcM (TidyEnv, Message)] - -- Innermost first. Monadic so that we have a chance - -- to deal with bound type variables just before error - -- message construction +type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, Message)) + -- Monadic so that we have a chance + -- to deal with bound type variables just before error + -- message construction + + -- Bool: True <=> this is a landmark context; do not + -- discard it when trimming for display \end{code} @@ -505,62 +553,70 @@ type ErrCtxt = [TidyEnv -> TcM (TidyEnv, Message)] %* * %************************************************************************ -ImportAvails summarises what was imported from where, irrespective -of whether the imported things are actually used or not -It is used * when processing the export list - * when constructing usage info for the inteface file - * to identify the list of directly imported modules - for initialisation purposes and - for optimsed overlap checking of family instances - * when figuring out what things are really unused - \begin{code} +-- | 'ImportAvails' summarises what was imported from where, irrespective of +-- whether the imported things are actually used or not. It is used: +-- +-- * when processing the export list, +-- +-- * when constructing usage info for the interface file, +-- +-- * to identify the list of directly imported modules for initialisation +-- purposes and for optimised overlap checking of family instances, +-- +-- * when figuring out what things are really unused +-- data ImportAvails = ImportAvails { - imp_mods :: ModuleEnv (Module, [(ModuleName, Bool, SrcSpan)]), - -- Domain is all directly-imported modules - -- The ModuleName is what the module was imported as, e.g. in - -- import Foo as Bar - -- it is Bar. - -- Bool means: - -- True => import was "import Foo ()" - -- False => import was some other form - -- - -- We need the Module in the range because we can't get - -- the keys of a ModuleEnv - -- Used - -- (a) to help construct the usage information in - -- the interface file; if we import somethign we - -- need to recompile if the export version changes - -- (b) to specify what child modules to initialise - -- - -- We need a full ModuleEnv rather than a ModuleNameEnv - -- here, because we might be importing modules of the - -- same name from different packages. (currently not the case, - -- but might be in the future). + imp_mods :: ModuleEnv [(ModuleName, Bool, SrcSpan)], + -- ^ Domain is all directly-imported modules + -- The 'ModuleName' is what the module was imported as, e.g. in + -- @ + -- import Foo as Bar + -- @ + -- it is @Bar@. + -- + -- The 'Bool' means: + -- + -- - @True@ => import was @import Foo ()@ + -- + -- - @False@ => import was some other form + -- + -- Used + -- + -- (a) to help construct the usage information in the interface + -- file; if we import somethign we need to recompile if the + -- export version changes + -- + -- (b) to specify what child modules to initialise + -- + -- We need a full ModuleEnv rather than a ModuleNameEnv here, + -- because we might be importing modules of the same name from + -- different packages. (currently not the case, but might be in the + -- future). imp_dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface), - -- Home-package modules needed by the module being compiled - -- - -- It doesn't matter whether any of these dependencies - -- are actually *used* when compiling the module; they - -- are listed if they are below it at all. For - -- example, suppose M imports A which imports X. Then - -- compiling M might not need to consult X.hi, but X - -- is still listed in M's dependencies. + -- ^ Home-package modules needed by the module being compiled + -- + -- It doesn't matter whether any of these dependencies + -- are actually /used/ when compiling the module; they + -- are listed if they are below it at all. For + -- example, suppose M imports A which imports X. Then + -- compiling M might not need to consult X.hi, but X + -- is still listed in M's dependencies. imp_dep_pkgs :: [PackageId], - -- Packages needed by the module being compiled, whether - -- directly, or via other modules in this package, or via - -- modules imported from other packages. + -- ^ Packages needed by the module being compiled, whether directly, + -- or via other modules in this package, or via modules imported + -- from other packages. imp_orphs :: [Module], - -- Orphan modules below us in the import tree (and maybe - -- including us for imported modules) + -- ^ Orphan modules below us in the import tree (and maybe including + -- us for imported modules) imp_finsts :: [Module] - -- Family instance modules below us in the import tree (and - -- maybe including us for imported modules) + -- ^ Family instance modules below us in the import tree (and maybe + -- including us for imported modules) } mkModDeps :: [(ModuleName, IsBootInterface)] @@ -584,13 +640,12 @@ plusImportAvails (ImportAvails { imp_mods = mods2, imp_dep_mods = dmods2, imp_dep_pkgs = dpkgs2, imp_orphs = orphs2, imp_finsts = finsts2 }) - = ImportAvails { imp_mods = plusModuleEnv_C plus_mod mods1 mods2, + = ImportAvails { imp_mods = plusModuleEnv_C (++) mods1 mods2, imp_dep_mods = plusUFM_C plus_mod_dep dmods1 dmods2, imp_dep_pkgs = dpkgs1 `unionLists` dpkgs2, imp_orphs = orphs1 `unionLists` orphs2, imp_finsts = finsts1 `unionLists` finsts2 } where - plus_mod (m1, xs1) (_, xs2) = (m1, xs1 ++ xs2) plus_mod_dep (m1, boot1) (m2, boot2) = WARN( not (m1 == m2), (ppr m1 <+> ppr m2) $$ (ppr boot1 <+> ppr boot2) ) -- Check mod-names match @@ -611,261 +666,236 @@ data WhereFrom | ImportBySystem -- Non user import. instance Outputable WhereFrom where - ppr (ImportByUser is_boot) | is_boot = ptext SLIT("{- SOURCE -}") + ppr (ImportByUser is_boot) | is_boot = ptext (sLit "{- SOURCE -}") | otherwise = empty - ppr ImportBySystem = ptext SLIT("{- SYSTEM -}") + ppr ImportBySystem = ptext (sLit "{- SYSTEM -}") \end{code} %************************************************************************ %* * -\subsection[Inst-types]{@Inst@ types} + Wanted constraints + + These are forced to be in TcRnTypes because + TcLclEnv mentions WantedConstraints + WantedConstraint mentions CtLoc + CtLoc mentions ErrCtxt + ErrCtxt mentions TcM %* * v%************************************************************************ -An @Inst@ is either a dictionary, an instance of an overloaded -literal, or an instance of an overloaded value. We call the latter a -``method'' even though it may not correspond to a class operation. -For example, we might have an instance of the @double@ function at -type Int, represented by +\begin{code} +data Untouchables = NoUntouchables + | TouchableRange + Unique -- Low end + Unique -- High end + -- A TcMetaTyvar is *touchable* iff its unique u satisfies + -- u >= low + -- u < high + +instance Outputable Untouchables where + ppr NoUntouchables = ptext (sLit "No untouchables") + ppr (TouchableRange low high) = ptext (sLit "Touchable range:") <+> + ppr low <+> char '-' <+> ppr high + +isNoUntouchables :: Untouchables -> Bool +isNoUntouchables NoUntouchables = True +isNoUntouchables (TouchableRange {}) = False + +inTouchableRange :: Untouchables -> TcTyVar -> Bool +inTouchableRange NoUntouchables _ = True +inTouchableRange (TouchableRange low high) tv + = uniq >= low && uniq < high + where + uniq = varUnique tv - Method 34 doubleId [Int] origin +type WantedConstraints = Bag WantedConstraint -In addition to the basic Haskell variants of 'Inst's, they can now also -represent implication constraints 'forall tvs. (reft, given) => wanted' -and equality constraints 'co :: ty1 ~ ty2'. +data WantedConstraint + = WcEvVar WantedEvVar + | WcImplic Implication + -- ToDo: add literals, methods -NB: Equalities occur in two flavours: +-- EvVar defined in module Var.lhs: +-- Evidence variables include all *quantifiable* constraints +-- dictionaries +-- implicit parameters +-- coercion variables - (1) Dict {tci_pred = EqPred ty1 ty2} - (2) EqInst {tci_left = ty1, tci_right = ty2, tci_co = coe} +data WantedEvVar -- The sort of constraint over which one can lambda-abstract + = WantedEvVar + EvVar -- The variable itself; make a binding for it please + WantedLoc -- How the constraint arose in the first place + -- (used for error messages only) -The former arises from equalities in contexts, whereas the latter is used -whenever the type checker introduces an equality (e.g., during deferring -unification). +type WantedLoc = CtLoc CtOrigin +type GivenLoc = CtLoc SkolemInfo -I am not convinced that this duplication is necessary or useful! -=chak +data Implication + = Implic { + ic_untch :: Untouchables, -- Untouchables: unification variables + -- free in the environment + ic_env :: TcTypeEnv, -- The type environment + -- Used only when generating error messages + -- Generally, ic_untch is a superset of tvsof(ic_env) + -- However, we don't zonk ic_env when zonking the Implication + -- Instead we do that when generating a skolem-escape error message -\begin{code} -data Inst - = Dict { - tci_name :: Name, - tci_pred :: TcPredType, - tci_loc :: InstLoc - } + ic_skols :: TcTyVarSet, -- Introduced skolems + -- See Note [Skolems in an implication] - | ImplicInst { -- An implication constraint - -- forall tvs. (reft, given) => wanted - tci_name :: Name, - tci_tyvars :: [TcTyVar], -- Quantified type variables - -- Includes coercion variables - -- mentioned in tci_reft - tci_reft :: Refinement, - tci_given :: [Inst], -- Only Dicts and EqInsts - -- (no Methods, LitInsts, ImplicInsts) - tci_wanted :: [Inst], -- Only Dicts, EqInst, and ImplicInsts - -- (no Methods or LitInsts) - - tci_loc :: InstLoc - } - -- NB: the tci_given are not necessarily rigid, - -- although they will be if the tci_reft is non-trivial - -- NB: the tci_reft is already applied to tci_given and tci_wanted + ic_scoped :: [TcTyVar], -- List of scoped variables to be unified + -- bijectively to a subset of ic_tyvars + -- Note [Scoped pattern variable] - | Method { - tci_id :: TcId, -- The Id for the Inst + ic_given :: [EvVar], -- Given evidence variables + -- (order does not matter) - tci_oid :: TcId, -- The overloaded function - -- This function will be a global, local, or ClassOpId; - -- inside instance decls (only) it can also be an InstId! - -- The id needn't be completely polymorphic. - -- You'll probably find its name (for documentation purposes) - -- inside the InstOrigin + ic_wanted :: WantedConstraints, -- Wanted constraints - tci_tys :: [TcType], -- The types to which its polymorphic tyvars - -- should be instantiated. - -- These types must saturate the Id's foralls. + ic_binds :: EvBindsVar, -- Points to the place to fill in the + -- abstraction and bindings - tci_theta :: TcThetaType, - -- The (types of the) dictionaries to which the function - -- must be applied to get the method + ic_loc :: GivenLoc } - tci_loc :: InstLoc - } - -- INVARIANT 1: in (Method m f tys theta tau loc) - -- type of m = type of (f tys dicts(from theta)) - - -- INVARIANT 2: type of m must not be of form (Pred -> Tau) - -- Reason: two methods are considered equal if the - -- base Id matches, and the instantiating types - -- match. The TcThetaType should then match too. - -- This only bites in the call to tcInstClassOp in TcClassDcl.mkMethodBind - - | LitInst { - tci_name :: Name, - tci_lit :: HsOverLit Name, -- The literal from the occurrence site - -- INVARIANT: never a rebindable-syntax literal - -- Reason: tcSyntaxName does unification, and we - -- don't want to deal with that during tcSimplify, - -- when resolving LitInsts - - tci_ty :: TcType, -- The type at which the literal is used - tci_loc :: InstLoc - } +evVarsToWanteds :: WantedLoc -> [EvVar] -> WantedConstraints +evVarsToWanteds loc evs = listToBag [WcEvVar (WantedEvVar ev loc) | ev <- evs] - | EqInst { -- delayed unification of the form - -- co :: ty1 ~ ty2 - tci_left :: TcType, -- ty1 -- both types are... - tci_right :: TcType, -- ty2 -- ...free of boxes - tci_co :: Either -- co - TcTyVar -- - a wanted equation, with a hole, to be - -- filled with a witness for the equality; - -- for equation arising from deferring - -- unification, 'ty1' is the actual and - -- 'ty2' the expected type - Coercion, -- - a given equation, with a coercion - -- witnessing the equality; - -- a coercion that originates from a - -- signature or a GADT is a CoVar, but - -- after normalisation of coercions, they - -- can be arbitrary Coercions involving - -- constructors and pseudo-constructors - -- like sym and trans. - tci_loc :: InstLoc, - - tci_name :: Name -- Debugging help only: this makes it easier to - -- follow where a constraint is used in a morass - -- of trace messages! Unlike other Insts, it has - -- no semantic significance whatsoever. - } -\end{code} +wantedEvVarLoc :: WantedEvVar -> WantedLoc +wantedEvVarLoc (WantedEvVar _ loc) = loc -@Insts@ are ordered by their class/type info, rather than by their -unique. This allows the context-reduction mechanism to use standard finite -maps to do their stuff. It's horrible that this code is here, rather -than with the Avails handling stuff in TcSimplify +wantedEvVarToVar :: WantedEvVar -> EvVar +wantedEvVarToVar (WantedEvVar ev _) = ev -\begin{code} -instance Ord Inst where - compare = cmpInst - -instance Eq Inst where - (==) i1 i2 = case i1 `cmpInst` i2 of - EQ -> True - other -> False - -cmpInst d1@(Dict {}) d2@(Dict {}) = tci_pred d1 `tcCmpPred` tci_pred d2 -cmpInst (Dict {}) other = LT - -cmpInst (Method {}) (Dict {}) = GT -cmpInst m1@(Method {}) m2@(Method {}) = (tci_oid m1 `compare` tci_oid m2) `thenCmp` - (tci_tys m1 `tcCmpTypes` tci_tys m2) -cmpInst (Method {}) other = LT - -cmpInst (LitInst {}) (Dict {}) = GT -cmpInst (LitInst {}) (Method {}) = GT -cmpInst l1@(LitInst {}) l2@(LitInst {}) = (tci_lit l1 `compare` tci_lit l2) `thenCmp` - (tci_ty l1 `tcCmpType` tci_ty l2) -cmpInst (LitInst {}) other = LT - - -- Implication constraints are compared by *name* - -- not by type; that is, we make no attempt to do CSE on them -cmpInst (ImplicInst {}) (Dict {}) = GT -cmpInst (ImplicInst {}) (Method {}) = GT -cmpInst (ImplicInst {}) (LitInst {}) = GT -cmpInst i1@(ImplicInst {}) i2@(ImplicInst {}) = tci_name i1 `compare` tci_name i2 -cmpInst (ImplicInst {}) other = LT - - -- same for Equality constraints -cmpInst (EqInst {}) (Dict {}) = GT -cmpInst (EqInst {}) (Method {}) = GT -cmpInst (EqInst {}) (LitInst {}) = GT -cmpInst (EqInst {}) (ImplicInst {}) = GT -cmpInst i1@(EqInst {}) i2@(EqInst {}) = tci_name i1 `compare` tci_name i2 +wantedEvVarPred :: WantedEvVar -> PredType +wantedEvVarPred (WantedEvVar ev _) = evVarPred ev + +splitWanteds :: WantedConstraints -> (Bag WantedEvVar, Bag Implication) +splitWanteds wanted = partitionBagWith pick wanted + where + pick (WcEvVar v) = Left v + pick (WcImplic i) = Right i \end{code} +Note [Skolems in an implication] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The skolems in an implication are not there to perform a skolem escape +check. That happens because all the environment variables are in the +untouchables, and therefore cannot be unified with anything at all, +let alone the skolems. -%************************************************************************ -%* * -\subsection[Inst-collections]{LIE: a collection of Insts} -%* * -%************************************************************************ +Instead, ic_skols is used only when considering floating a constraint +outside the implication in TcSimplify.floatEqualities or +TcSimplify.approximateImplications + +Note [Scoped pattern variables] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + data T where K :: forall a,b. a -> b -> T + + ...(case x of K (p::c) (q::d) -> ...)... + +We create fresh MetaTvs for c,d, and later check that they are +bound bijectively to the skolems we created for a,b. So the +implication constraint looks like + ic_skols = {a',b'} -- Skolem tvs created from a,b + ic_scoped = {c',d'} -- Meta tvs created from c,d \begin{code} --- FIXME: Rename this. It clashes with (Located (IE ...)) -type LIE = Bag Inst - -isEmptyLIE = isEmptyBag -emptyLIE = emptyBag -unitLIE inst = unitBag inst -mkLIE insts = listToBag insts -plusLIE lie1 lie2 = lie1 `unionBags` lie2 -plusLIEs lies = unionManyBags lies -lieToList = bagToList -listToLIE = listToBag - -consLIE inst lie = lie `snocBag` inst --- Putting the new Inst at the *end* of the bag is a half-hearted attempt --- to ensure that we tend to report the *leftmost* type-constraint error --- E.g. f :: [a] --- f = [1,2,3] --- we'd like to complain about the '1', not the '3'. --- --- "Half-hearted" because the rest of the type checker makes no great --- claims for retaining order in the constraint set. Still, this --- seems to improve matters slightly. Exampes: mdofail001, tcfail015 -\end{code} +emptyWanteds :: WantedConstraints +emptyWanteds = emptyBag +andWanteds :: WantedConstraints -> WantedConstraints -> WantedConstraints +andWanteds = unionBags + +extendWanteds :: WantedConstraints -> WantedConstraint -> WantedConstraints +extendWanteds = snocBag +\end{code} + +\begin{code} +pprEvVars :: [EvVar] -> SDoc -- Print with their types +pprEvVars ev_vars = vcat (map pprEvVarWithType ev_vars) + +pprEvVarTheta :: [EvVar] -> SDoc +pprEvVarTheta ev_vars = pprTheta (map evVarPred ev_vars) + +pprEvVarWithType :: EvVar -> SDoc +pprEvVarWithType v = ppr v <+> dcolon <+> pprPred (evVarPred v) + +pprWantedsWithLocs :: Bag WantedConstraint -> SDoc +pprWantedsWithLocs = foldrBag (($$) . pprWantedWithLoc) empty + +pprWantedWithLoc :: WantedConstraint -> SDoc +pprWantedWithLoc (WcImplic i) = ppr i +pprWantedWithLoc (WcEvVar v) = pprWantedEvVarWithLoc v + +instance Outputable WantedConstraint where + ppr (WcEvVar v) = ppr v + ppr (WcImplic i) = ppr i + +-- Adding -ferror-spans makes the output more voluminous +instance Outputable WantedEvVar where + ppr wev | opt_ErrorSpans = pprWantedEvVarWithLoc wev + | otherwise = pprWantedEvVar wev + +pprWantedEvVarWithLoc, pprWantedEvVar :: WantedEvVar -> SDoc +pprWantedEvVarWithLoc (WantedEvVar v loc) = hang (pprEvVarWithType v) + 2 (pprArisingAt loc) +pprWantedEvVar (WantedEvVar v _) = pprEvVarWithType v + +instance Outputable Implication where + ppr (Implic { ic_untch = untch, ic_skols = skols, ic_given = given + , ic_wanted = wanted, ic_binds = binds, ic_loc = loc }) + = ptext (sLit "Implic") <+> braces + (sep [ ptext (sLit "Untouchables = ") <+> ppr untch + , ptext (sLit "Skolems = ") <+> ppr skols + , ptext (sLit "Given = ") <+> pprEvVars given + , ptext (sLit "Wanted = ") <+> ppr wanted + , ptext (sLit "Binds = ") <+> ppr binds + , pprSkolInfo (ctLocOrigin loc) + , ppr (ctLocSpan loc) ]) +\end{code} %************************************************************************ %* * -\subsection[Inst-origin]{The @InstOrigin@ type} + CtLoc, CtOrigin %* * %************************************************************************ -The @InstOrigin@ type gives information about where a dictionary came from. -This is important for decent error message reporting because dictionaries -don't appear in the original source code. Doubtless this type will evolve... - -It appears in TcMonad because there are a couple of error-message-generation -functions that deal with it. +The 'CtLoc' and 'CtOrigin' types gives information about where a +*wanted constraint* came from. This is important for decent error +message reporting because dictionaries don't appear in the original +source code. Doubtless this type will evolve... \begin{code} ------------------------------------------- -data InstLoc = InstLoc InstOrigin SrcSpan ErrCtxt +data CtLoc orig = CtLoc orig SrcSpan [ErrCtxt] -instLoc :: Inst -> InstLoc -instLoc inst = tci_loc inst +ctLocSpan :: CtLoc o -> SrcSpan +ctLocSpan (CtLoc _ s _) = s -instSpan :: Inst -> SrcSpan -instSpan wanted = instLocSpan (instLoc wanted) +ctLocOrigin :: CtLoc o -> o +ctLocOrigin (CtLoc o _ _) = o -instLocSpan :: InstLoc -> SrcSpan -instLocSpan (InstLoc _ s _) = s +setCtLocOrigin :: CtLoc o -> o' -> CtLoc o' +setCtLocOrigin (CtLoc _ s c) o = CtLoc o s c -instLocOrigin :: InstLoc -> InstOrigin -instLocOrigin (InstLoc o _ _) = o +pprArising :: CtOrigin -> SDoc +pprArising (TypeEqOrigin {}) = empty +pprArising orig = text "arising from" <+> ppr orig -pprInstArising :: Inst -> SDoc -pprInstArising loc = ptext SLIT("arising from") <+> pprInstLoc (tci_loc loc) - -pprInstLoc :: InstLoc -> SDoc -pprInstLoc (InstLoc orig span _) = sep [ppr orig, text "at" <+> ppr span] +pprArisingAt :: CtLoc CtOrigin -> SDoc +pprArisingAt (CtLoc o s _) = sep [pprArising o, text "at" <+> ppr s] ------------------------------------------- -data InstOrigin - = SigOrigin SkolemInfo -- Pattern, class decl, inst decl etc; - -- Places that bind type variables and introduce - -- available constraints - - | IPBindOrigin (IPName Name) -- Binding site of an implicit parameter +-- CtOrigin gives the origin of *wanted* constraints +data CtOrigin + = OccurrenceOf Name -- Occurrence of an overloaded identifier + | AppOrigin -- An application of some kind - ------------------------------------------------------- - -- The rest are all occurrences: Insts that are 'wanted' - ------------------------------------------------------- - | OccurrenceOf Name -- Occurrence of an overloaded identifier | SpecPragOrigin Name -- Specialisation pragma for identifier + | TypeEqOrigin EqOrigin + | IPOccOrigin (IPName Name) -- Occurrence of an implicit parameter | LiteralOrigin (HsOverLit Name) -- Occurrence of a literal @@ -873,43 +903,55 @@ data InstOrigin | ArithSeqOrigin (ArithSeqInfo Name) -- [x..], [x..y] etc | PArrSeqOrigin (ArithSeqInfo Name) -- [:x..y:] and [:x,y..z:] + | SectionOrigin | TupleOrigin -- (..,..) - - | InstSigOrigin -- A dict occurrence arising from instantiating - -- a polymorphic type during a subsumption check - | ExprSigOrigin -- e :: ty + | PatSigOrigin -- p :: ty + | PatOrigin -- Instantiating a polytyped pattern at a constructor | RecordUpdOrigin | ViewPatOrigin - | InstScOrigin -- Typechecking superclasses of an instance declaration + + | ScOrigin -- Typechecking superclasses of an instance declaration | DerivOrigin -- Typechecking deriving | StandAloneDerivOrigin -- Typechecking stand-alone deriving | DefaultOrigin -- Typechecking a default decl | DoOrigin -- Arising from a do expression | ProcOrigin -- Arising from a proc expression - | ImplicOrigin SDoc -- An implication constraint - | EqOrigin -- A type equality - -instance Outputable InstOrigin where - ppr (OccurrenceOf name) = hsep [ptext SLIT("a use of"), quotes (ppr name)] - ppr (SpecPragOrigin name) = hsep [ptext SLIT("a specialisation pragma for"), quotes (ppr name)] - ppr (IPOccOrigin name) = hsep [ptext SLIT("a use of implicit parameter"), quotes (ppr name)] - ppr (IPBindOrigin name) = hsep [ptext SLIT("a binding for implicit parameter"), quotes (ppr name)] - ppr RecordUpdOrigin = ptext SLIT("a record update") - ppr ExprSigOrigin = ptext SLIT("an expression type signature") - ppr ViewPatOrigin = ptext SLIT("a view pattern") - ppr (LiteralOrigin lit) = hsep [ptext SLIT("the literal"), quotes (ppr lit)] - ppr (ArithSeqOrigin seq) = hsep [ptext SLIT("the arithmetic sequence"), quotes (ppr seq)] - ppr (PArrSeqOrigin seq) = hsep [ptext SLIT("the parallel array sequence"), quotes (ppr seq)] - ppr TupleOrigin = ptext SLIT("a tuple") - ppr NegateOrigin = ptext SLIT("a use of syntactic negation") - ppr InstScOrigin = ptext SLIT("the superclasses of an instance declaration") - ppr DerivOrigin = ptext SLIT("the 'deriving' clause of a data type declaration") - ppr StandAloneDerivOrigin = ptext SLIT("a 'deriving' declaration") - ppr DefaultOrigin = ptext SLIT("a 'default' declaration") - ppr DoOrigin = ptext SLIT("a do statement") - ppr ProcOrigin = ptext SLIT("a proc expression") - ppr (ImplicOrigin doc) = doc - ppr (SigOrigin info) = pprSkolInfo info - ppr EqOrigin = ptext SLIT("a type equality") + | AnnOrigin -- An annotation + +data EqOrigin + = UnifyOrigin + { uo_actual :: TcType + , uo_expected :: TcType } + +instance Outputable CtOrigin where + ppr orig = pprO orig + +pprO :: CtOrigin -> SDoc +pprO (OccurrenceOf name) = hsep [ptext (sLit "a use of"), quotes (ppr name)] +pprO AppOrigin = ptext (sLit "an application") +pprO (SpecPragOrigin name) = hsep [ptext (sLit "a specialisation pragma for"), quotes (ppr name)] +pprO (IPOccOrigin name) = hsep [ptext (sLit "a use of implicit parameter"), quotes (ppr name)] +pprO RecordUpdOrigin = ptext (sLit "a record update") +pprO ExprSigOrigin = ptext (sLit "an expression type signature") +pprO PatSigOrigin = ptext (sLit "a pattern type signature") +pprO PatOrigin = ptext (sLit "a pattern") +pprO ViewPatOrigin = ptext (sLit "a view pattern") +pprO (LiteralOrigin lit) = hsep [ptext (sLit "the literal"), quotes (ppr lit)] +pprO (ArithSeqOrigin seq) = hsep [ptext (sLit "the arithmetic sequence"), quotes (ppr seq)] +pprO (PArrSeqOrigin seq) = hsep [ptext (sLit "the parallel array sequence"), quotes (ppr seq)] +pprO SectionOrigin = ptext (sLit "an operator section") +pprO TupleOrigin = ptext (sLit "a tuple") +pprO NegateOrigin = ptext (sLit "a use of syntactic negation") +pprO ScOrigin = ptext (sLit "the superclasses of an instance declaration") +pprO DerivOrigin = ptext (sLit "the 'deriving' clause of a data type declaration") +pprO StandAloneDerivOrigin = ptext (sLit "a 'deriving' declaration") +pprO DefaultOrigin = ptext (sLit "a 'default' declaration") +pprO DoOrigin = ptext (sLit "a do statement") +pprO ProcOrigin = ptext (sLit "a proc expression") +pprO (TypeEqOrigin eq) = ptext (sLit "an equality") <+> ppr eq +pprO AnnOrigin = ptext (sLit "an annotation") + +instance Outputable EqOrigin where + ppr (UnifyOrigin t1 t2) = ppr t1 <+> char '~' <+> ppr t2 \end{code}