Add non-recursive let-bindings for types

[ghc-hetmet.git] / compiler / coreSyn / CoreSyn.lhs

diff --git a/compiler/coreSyn/CoreSyn.lhs b/compiler/coreSyn/CoreSyn.lhs
index f732a95..ac32bc2 100644 (file)
--- a/compiler/coreSyn/CoreSyn.lhs
+++ b/compiler/coreSyn/CoreSyn.lhs
@@ -20,9 +20,8 @@ module CoreSyn (
        isTyVar, isId, cmpAltCon, cmpAlt, ltAlt,
        bindersOf, bindersOfBinds, rhssOfBind, rhssOfAlts, 
        collectBinders, collectTyBinders, collectValBinders, collectTyAndValBinders,
-       collectArgs, 
-       coreExprCc,
-       flattenBinds, 
+       collectArgs, coreExprCc,
+       mkTyBind, flattenBinds, 
 
        isValArg, isTypeArg, valArgCount, valBndrCount, isRuntimeArg, isRuntimeVar,
 
@@ -150,12 +149,23 @@ Invariant: The list of alternatives is ALWAYS EXHAUSTIVE,
 
 
 Note [CoreSyn let goal]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~
 * The simplifier tries to ensure that if the RHS of a let is a constructor
   application, its arguments are trivial, so that the constructor can be
   inlined vigorously.
 
 
+Note [Type let]
+~~~~~~~~~~~~~~~
+We allow a *non-recursive* let to bind a type variable, thus
+       Let (NonRec tv (Type ty)) body
+This can be very convenient for postponing type substitutions until
+the next run of the simplifier.
+
+At the moment, the rest of the compiler only deals with type-let
+in a Let expression, rather than at top level.  We may want to revist
+this choice.
+
 \begin{code}
 data Note
   = SCC CostCentre
@@ -501,6 +511,11 @@ mkCast e co = Cast e co
 %************************************************************************
 
 \begin{code}
+mkTyBind :: TyVar -> Type -> CoreBind
+mkTyBind tv ty      = NonRec tv (Type ty)
+       -- Note [Type let]
+       -- A non-recursive let can bind a type variable
+
 bindersOf  :: Bind b -> [b]
 bindersOf (NonRec binder _) = [binder]
 bindersOf (Rec pairs)       = [binder | (binder, _) <- pairs]