#7179: Non-unique variable bindings in the Core
-------------------------------+--------------------------------------------
Reporter: stephen70edwards | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: External Core | Version: 7.4.1
Resolution: wontfix | Keywords: CoreSyn Unique
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Other | Difficulty: Unknown
Testcase: | Blockedby:
Blocking: | Related:
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Changes (by simonpj):
* status: new => closed
* difficulty: => Unknown
* resolution: => wontfix
Comment:
I'm afraid Core makes no claim to uniqueness. In general, the Simplifier
tends to remove shadowing, so it will tend not to produce this:
{{{
(\x_3. \x_3. ...x_3...)
}}}
But that's only to make life easier for the Simplifier itself; it's not a
guarantee. And no other pass is required to ensure uniqueness.
The simplifier does not even try to eliminate uses of the same binder in
different sub-expressions (your example).
These would be hard invariants to maintain. If you want to establish such
an invariant, just before you do something else. you can write a pass that
does so.
I recently added this comment to `CoreSyn.lhs`:
{{{
Note [Shadowing]
~~~~~~~~~~~~~~~~
While various passes attempt to rename on-the-fly in a manner that
avoids "shadowing" (thereby simplifying downstream optimizations),
neither the simplifier nor any other pass GUARANTEES that shadowing is
avoided. Thus, all passes SHOULD work fine even in the presence of
arbitrary shadowing in their inputs.
In particular, scrutinee variables `x` in expressions of the form
`Case e x t` are often renamed to variables with a prefix
"wild_". These "wild" variables may appear in the body of the
case-expression, and further, may be shadowed within the body.
}}}
If you would like to modify it, or add some more clarification, can you
suggest some words and I'll add them?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7179#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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