Id typecheck/TcHsSyn
\begin{code}
-{-# OPTIONS_GHC -w #-}
+{-# 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/WorkingConventions#Warnings
+-- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
-- for details
module HsUtils where
we want to generate bindings for x,y but not for dictionaries bound by
C. (The type checker ensures they would not be used.)
+Desugaring of arrow case expressions needs these bindings (see DsArrows
+and arrowcase1), but SPJ (Jan 2007) says it's safer for it to use its
+own pat-binder-collector:
+
+Here's the problem. Consider
+
+data T a where
+ C :: Num a => a -> Int -> T a
+
+f ~(C (n+1) m) = (n,m)
+
+Here, the pattern (C (n+1)) binds a hidden dictionary (d::Num a),
+and *also* uses that dictionary to match the (n+1) pattern. Yet, the
+variables bound by the lazy pattern are n,m, *not* the dictionary d.
+So in mkSelectorBinds in DsUtils, we want just m,n as the variables bound.
+
\begin{code}
collectSigTysFromPats :: [InPat name] -> [LHsType name]
collectSigTysFromPats pats = foldr collect_lpat [] pats