On Thu, Sep 20, 2007 at 06:13:38AM +0400, Victor Nazarov wrote:
I still have some questions regarding the GHC internals. There is a description of STG language in the "Making a Fast Curry: Push/Enter vs. Eval/Apply for Higher-oder Languages" (2004) by Simon Marlow and Simon Peyton Jones paper. In this description the constructor application (CONS closure) can only appear on the right hand side of the bindings. This is totally reasonable if "let" is the only construct that allocates objects. But in the GHC's StgSyn.hs any expression can be constructor application. How does constructor applications are compiled? Are they implicitly transformed to let? For example:
f = let g = (THUNK h x) in (CONS g y)
Is this exactly the same as (right variant following the paper)
f = let g = (THUNK h x) in let freshvar = (CONS g y) in freshvar
?
And the second question is how does constructor tag is passed to case when non-vector return is used? In register? In constructor closure? Are there any cases when closure is not build for constructor application? What the case binder is bound to if there is no closure for constructor application?
The StgSyn type *declaration* allows this stuff. In actual use, it's always kept in, and expected to be in, A-normal form. The Simons have said that much of GHC is sadly written as though it was written in a dynamically typed language and then shoddily ported to Haskell; types do NOT describe ghc's data, to borrow one of Conor's catchphrases. (I can't find the original) Stefan