[commit: ghc] cardinality: more changes in the analyzer documented (50ac991)

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

On branch  : cardinality

http://hackage.haskell.org/trac/ghc/changeset/50ac991d208c366b5d654a646d756136de88d171

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

commit 50ac991d208c366b5d654a646d756136de88d171
Author: Ilya Sergey <[email protected]>
Date:   Mon Sep 24 13:56:18 2012 +0100

    more changes in the analyzer documented

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

 compiler/stranal/DmdAnal.lhs |   33 +++++++++++++++++++++++++++++++--
 1 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/compiler/stranal/DmdAnal.lhs b/compiler/stranal/DmdAnal.lhs
index 6eb1970..8b295bd 100644
--- a/compiler/stranal/DmdAnal.lhs
+++ b/compiler/stranal/DmdAnal.lhs
@@ -79,6 +79,7 @@ dmdAnalTopBind sigs (Rec pairs)
 		-- We get two iterations automatically
 		-- c.f. the NonRec case above
 
+-- See Note [Analysing lambdas at right-hand side]
 data RhsFlag = MayBeRhsLambda | MereExpr
 
 \end{code}
@@ -184,8 +185,6 @@ dmdAnal rhs_flag env dmd (Lam var body)
     (lam_ty, Lam var' body')
 
   | Just (body_dmd, Many) <- peelCallDmd dmd	
-  -- A call demand, also a one-shot lambda
-  -- see Note [Analyzing with lazy demand and lambdas]
   = let	
         env'		 = extendSigsWithLam env var
 	(body_ty, body') = dmdAnal MereExpr env' body_dmd body
@@ -342,6 +341,36 @@ dmdAnalAlt env dmd (con,bndrs,rhs)
 
 \end{code}
 
+Note [Analysing lambdas at right-hand side]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is safe to analyze a lambda-expression on a right-hand-side of a
+let-binding with a usage demand C1(C1...(U)), where the number of C1s
+is the same as "visible" arity of the right-hand side. However, this
+poses a problem when markin lambda s on-shot. Indeed, both these lambdas:
+
+let g = \x -> x + 1 in ...
+
+and 
+
+(\x -> x + 1) 5
+
+will be marked as "one-shot", whereas only the latter one is. A
+let-bound lambda can be, of course, invoked multiple times, and we
+cannot state it to be on-shot just lookting at the definition
+site. Therefore, we pass an extra flag to the analysis:
+
+data RhsFlag = MayBeRhsLambda | MereExpr
+
+in order to ditinguish, if the currently analyzed expression is a
+(possibly nested) lambda, located *immediately* at RHS of som binding
+(then the one-shot annotation is not assigned) or just an arbitrary
+lambda expression somewhere, e.g.
+
+build g = g (:) []
+build (\x y -> x () y) -- this lambda is one-shot
+
+
 Note [Analyzing with lazy demand and lambdas]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~