[commit: ghc] master: Improve pretty-printing of strict lets with -dppr-case-as-let (41288a1)

[Simon Peyton Jones]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/41288a1d37047b4046f43bb88f8aaf6bdefd8183

>---------------------------------------------------------------

commit 41288a1d37047b4046f43bb88f8aaf6bdefd8183
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Wed Feb 13 17:07:26 2013 +0000

    Improve pretty-printing of strict lets with -dppr-case-as-let

>---------------------------------------------------------------

 compiler/coreSyn/PprCore.lhs |   29 ++++++++++++++++++-----------
 1 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/compiler/coreSyn/PprCore.lhs b/compiler/coreSyn/PprCore.lhs
index fa1cde9..0ead297 100644
--- a/compiler/coreSyn/PprCore.lhs
+++ b/compiler/coreSyn/PprCore.lhs
@@ -157,17 +157,13 @@ ppr_expr add_par expr@(App {})
 ppr_expr add_par (Case expr var ty [(con,args,rhs)])
   = sdocWithDynFlags $ \dflags ->
     if gopt Opt_PprCaseAsLet dflags
-    then add_par $
-         sep [sep    [ ptext (sLit "let")
-                             <+> char '{'
-                             <+> ppr_case_pat con args
-                             <+> ptext (sLit "~")
-                             <+> ppr_bndr var
-                     , ptext (sLit "<-")
-                             <+> ppr_expr id expr
-                     , char '}'
-                             <+> ptext (sLit "in")
-                     ]
+    then add_par $  -- See Note [Print case as let]
+         sep [ sep [ ptext (sLit "let! {") 
+                     <+> ppr_case_pat con args
+                     <+> ptext (sLit "~")
+                     <+> ppr_bndr var
+                   , ptext (sLit "<-") <+> ppr_expr id expr
+                     <+> ptext (sLit "} in") ]
              , pprCoreExpr rhs
              ]
     else add_par $
@@ -258,6 +254,17 @@ pprArg (Coercion co) = ptext (sLit "@~") <+> pprParendCo co
 pprArg expr          = pprParendExpr expr
 \end{code}
 
+Note [Print case as let]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Single-branch case expressions are very common:
+   case x of y { I# x' -> 
+   case p of q { I# p' -> ... } }
+These are, in effect, just strict let's, with pattern matching.
+With -dppr-case-as-let we print them as such:
+   let! { I# x' ~ y <- x } in
+   let! { I# p' ~ q <- p } in ...
+
+ 
 Other printing bits-and-bobs used with the general @pprCoreBinding@
 and @pprCoreExpr@ functions.