swap <[]> and <{}> syntax

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

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index a0a229f..19ac62f 100644 (file)
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -589,12 +589,16 @@ exprIsCheap' good_app other_expr  -- Applications and variables
     go _ _ = False
  
     --------------
-    go_pap args = all exprIsTrivial args
-       -- For constructor applications and primops, check that all
-       -- the args are trivial.  We don't want to treat as cheap, say,
-       --      (1:2:3:4:5:[])
-       -- We'll put up with one constructor application, but not dozens
-       
+    go_pap args = all (exprIsCheap' good_app) args
+        -- Used to be "all exprIsTrivial args" due to concerns about
+        -- duplicating nested constructor applications, but see #4978.
+       -- The principle here is that 
+       --    let x = a +# b in c *# x
+        -- should behave equivalently to
+        --    c *# (a +# b)
+        -- Since lets with cheap RHSs are accepted, 
+        -- so should paps with cheap arguments
+
     --------------
     go_primop op args = primOpIsCheap op && all (exprIsCheap' good_app) args
        -- In principle we should worry about primops