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Diffstat (limited to 'compiler/prelude/TysWiredIn.hs')
| -rw-r--r-- | compiler/prelude/TysWiredIn.hs | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs index b1ba7bf4b2..de7ec7ec81 100644 --- a/compiler/prelude/TysWiredIn.hs +++ b/compiler/prelude/TysWiredIn.hs @@ -260,6 +260,27 @@ eqTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "~") eqTyConK eqDataConName = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "Eq#") eqDataConKey eqDataCon eqSCSelIdName = mkWiredInIdName gHC_TYPES (fsLit "eq_sel") eqSCSelIdKey eqSCSelId +{- Note [eqTyCon (~) is built-in syntax] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The (~) type operator used in equality constraints (a~b) is considered built-in +syntax. This has a few consequences: + +* The user is not allowed to define their own type constructors with this name: + + ghci> class a ~ b + <interactive>:1:1: error: Illegal binding of built-in syntax: ~ + +* Writing (a ~ b) does not require enabling -XTypeOperators. It does, however, + require -XGADTs or -XTypeFamilies. + +* The (~) type operator is always in scope. It doesn't need to be be imported, + and it cannot be hidden. + +* We have a bunch of special cases in the compiler to arrange all of the above. + +There's no particular reason for (~) to be special, but fixing this would be a +breaking change. +-} eqTyCon_RDR :: RdrName eqTyCon_RDR = nameRdrName eqTyConName |