On Thu, Mar 14, 2013 at 10:43:14PM +0000, Simon Peyton-Jones wrote:
| -----Original Message----- | From: Tom Ellis | The -ddump-simpl output below doesn't contain a case corresponding to the | seq in sum', but the -ddump-prep does. Isn't the output from simpl the | input to prep? If so, where does the case reappear from? If not, how are | simpl and prep related? | | It seems to have something to do with "Str=DmdType SS" but I don't | understand. This seems to come from the IdInfo on the Id which is the | binder "Test.sum'" but [1] says that this information is optional so it | seems strange that such crucial information would be encoded there.
Check out http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/HscMain and the notes at the top of http://darcs.haskell.org/ghc/compiler/coreSyn/CorePrep.lhs
Thanks Simon. That's lovely code and easy to read. I see that the decision between let and case is made based on the idDemandInfo which specifies whether a value is wanted strictly or not. I presume that the explicit case is removed in the Simpl phase because it is easier for optimisations to work on the Demand value than explicit case statements. Tom