#4937: Remove indirections caused by sum types, such as Maybe
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Reporter: tibbe | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler | Version: 7.0.1
Resolution: | Keywords:
Testcase: | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: None/Unknown |
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Comment(by simonmar):
Replying to [comment:11 tibbe]:
> Couldn't we represent a sum of pointers (e.g. `data Either a b = Left a
| Right b`) as a new heap object, a sum-pointer object, which uses tag
bits to distinguish the constructors?
But `Either` already is a heap object, and we have tag bits to distinguish
`Left` from `Right` in the pointer itself.
> This, together with the optimization to distinguish nullary
constructors, would allow this data type to be represented with a single
pointer:
>
> {{{
> data Elem k v = Null | Key k | Value v | Subtree (Tree k v)
> }}}
So think about the representation of `Elem (Elem k v) v'`. You can't
represent that with a single pointer, because you need two sets of tag
bits.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4937#comment:12>
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