SYB gone wild - mysterious <<loop>> in code derived from an syb-with-class example
(From http://www.haskell.org/pipermail/haskell-cafe/2009-December/070193.html)
In the attached code,
- if you load the code into ghci and evaluate e it will hang, but (defaultValueD dict) :: Expression returns fine
- if you change the gunfold instance for Proposition to, error "gunfold" it stops hanging -- even though this code is never called.
- if you change, ( Data ctx [Expression], Sat (ctx Expression) => Data ctx Expression, to (Data ctx Expression, ....) => ... it stops hanging.
If someone could explain why each of these cases perform as they do, that would be awesome! Right now it is a big mystery to me.. e calls dict .. and there is only one instance of dict available, which should call error right away. I can't see how something could get in the way there...
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.12.1 RC1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |
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