| HsCase (LHsExpr id)
(MatchGroup id)
- | HsIf (LHsExpr id) -- predicate
+ | HsIf (Maybe (SyntaxExpr id)) -- cond function
+ -- Nothing => use the built-in 'if'
+ -- See Note [Rebindable if]
+ (LHsExpr id) -- predicate
(LHsExpr id) -- then part
(LHsExpr id) -- else part
-- pasted back in by the desugarer
\end{code}
-A @Dictionary@, unless of length 0 or 1, becomes a tuple. A
-@ClassDictLam dictvars methods expr@ is, therefore:
-\begin{verbatim}
-\ x -> case x of ( dictvars-and-methods-tuple ) -> expr
-\end{verbatim}
+Note [Rebindable if]
+~~~~~~~~~~~~~~~~~~~~
+The rebindable syntax for 'if' is a bit special, because when
+rebindable syntax is *off* we do not want to treat
+ (if c then t else e)
+as if it was an application (ifThenElse c t e). Why not?
+Because we allow an 'if' to return *unboxed* results, thus
+ if blah then 3# else 4#
+whereas that would not be possible using a all to a polymorphic function
+(because you can't call a polymorphic function at an unboxed type).
+
+So we use Nothing to mean "use the old built-in typing rule".
\begin{code}
instance OutputableBndr id => Outputable (HsExpr id) where
nest 2 (pprMatches (CaseAlt `asTypeOf` idType exprType) matches <+> char '}') ]
where idType :: HsExpr id -> HsMatchContext id; idType = undefined
-ppr_expr (HsIf e1 e2 e3)
+ppr_expr (HsIf _ e1 e2 e3)
= sep [hsep [ptext (sLit "if"), nest 2 (ppr e1), ptext (sLit "then")],
nest 4 (ppr e2),
ptext (sLit "else"),
[Match id] -- bodies are HsCmd's
SrcLoc
- | HsIf (HsExpr id) -- predicate
+ | HsIf (Maybe (SyntaxExpr id)) -- cond function
+ (HsExpr id) -- predicate
(HsCmd id) -- then part
(HsCmd id) -- else part
SrcLoc