Default.hs

{-
(c) The University of Glasgow 2006
(c) The AQUA Project, Glasgow University, 1993-1998

-}
{-# LANGUAGE TypeFamilies #-}

-- | Typechecking @default@ declarations
module GHC.Tc.Gen.Default ( tcDefaults ) where

import GHC.Prelude

import GHC.Hs
import GHC.Core.Class
import GHC.Core.Type ( typeKind )
import GHC.Types.Var( tyVarKind )
import GHC.Tc.Utils.Monad
import GHC.Tc.Utils.Env
import GHC.Tc.Gen.HsType
import GHC.Tc.Utils.Zonk
import GHC.Tc.Solver
import GHC.Tc.Validity
import GHC.Tc.Utils.TcType
import GHC.Builtin.Names
import GHC.Types.SrcLoc
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Data.FastString
import qualified GHC.LanguageExtensions as LangExt

tcDefaults :: [LDefaultDecl GhcRn]
           -> TcM (Maybe [Type])    -- Defaulting types to heave
                                    -- into Tc monad for later use
                                    -- in Disambig.

tcDefaults []
  = getDeclaredDefaultTys       -- No default declaration, so get the
                                -- default types from the envt;
                                -- i.e. use the current ones
                                -- (the caller will put them back there)
        -- It's important not to return defaultDefaultTys here (which
        -- we used to do) because in a TH program, tcDefaults [] is called
        -- repeatedly, once for each group of declarations between top-level
        -- splices.  We don't want to carefully set the default types in
        -- one group, only for the next group to ignore them and install
        -- defaultDefaultTys

tcDefaults [L _ (DefaultDecl _ [])]
  = return (Just [])            -- Default declaration specifying no types

tcDefaults [L locn (DefaultDecl _ mono_tys)]
  = setSrcSpan (locA locn)              $
    addErrCtxt defaultDeclCtxt          $
    do  { ovl_str   <- xoptM LangExt.OverloadedStrings
        ; ext_deflt <- xoptM LangExt.ExtendedDefaultRules
        ; num_class    <- tcLookupClass numClassName
        ; deflt_str <- if ovl_str
                       then mapM tcLookupClass [isStringClassName]
                       else return []
        ; deflt_interactive <- if ext_deflt
                               then mapM tcLookupClass interactiveClassNames
                               else return []
        ; let deflt_clss = num_class : deflt_str ++ deflt_interactive

        ; tau_tys <- mapAndReportM (tc_default_ty deflt_clss) mono_tys

        ; return (Just tau_tys) }

tcDefaults decls@(L locn (DefaultDecl _ _) : _)
  = setSrcSpan (locA locn) $
    failWithTc (dupDefaultDeclErr decls)


tc_default_ty :: [Class] -> LHsType GhcRn -> TcM Type
tc_default_ty deflt_clss hs_ty
 = do   { ty <- solveEqualities "tc_default_ty" $
                tcInferLHsType hs_ty
        ; ty <- zonkTcTypeToType ty   -- establish Type invariants
        ; checkValidType DefaultDeclCtxt ty

        -- Check that the type is an instance of at least one of the deflt_clss
        ; oks <- mapM (check_instance ty) deflt_clss
        ; checkTc (or oks) (badDefaultTy ty deflt_clss)
        ; return ty }

check_instance :: Type -> Class -> TcM Bool
-- Check that ty is an instance of cls
-- We only care about whether it worked or not; return a boolean
-- This checks that  cls :: k -> Constraint
-- with just one argument and no polymorphism; if we need to add
-- polymorphism we can make it more complicated.  For now we are
-- concerned with classes like
--    Num      :: Type -> Constraint
--    Foldable :: (Type->Type) -> Constraint
check_instance ty cls
  | [cls_tv] <- classTyVars cls
  , tyVarKind cls_tv `tcEqType` typeKind ty
  = simplifyDefault [mkClassPred cls [ty]]
  | otherwise
  = return False

defaultDeclCtxt :: SDoc
defaultDeclCtxt = text "When checking the types in a default declaration"

dupDefaultDeclErr :: [LDefaultDecl GhcRn] -> SDoc
dupDefaultDeclErr (L _ (DefaultDecl _ _) : dup_things)
  = hang (text "Multiple default declarations")
       2 (vcat (map pp dup_things))
  where
    pp :: LDefaultDecl GhcRn -> SDoc
    pp (L locn (DefaultDecl _ _))
      = text "here was another default declaration" <+> ppr (locA locn)
dupDefaultDeclErr [] = panic "dupDefaultDeclErr []"

badDefaultTy :: Type -> [Class] -> SDoc
badDefaultTy ty deflt_clss
  = hang (text "The default type" <+> quotes (ppr ty) <+> ptext (sLit "is not an instance of"))
       2 (foldr1 (\a b -> a <+> text "or" <+> b) (map (quotes. ppr) deflt_clss))