non-terminating instances involving FDs
GHC accepts the following (tc109) with -fglasgow-exts:
class P a
class R a b | b->a
instance (P a,R a b) => P [b]However as pointed out in the FD-CHR paper (example 15), adding the instance
instance R [[a]] [a]will cause non-termination. Such instances (context contains variables not in the head, even if determined by variables in the head) should be forbidden without -fallow-undecidable-instances. (This is the second part of the Bound Variable Condition in the above paper). Similar cases are tc118 and tc125.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.4.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | Unknown |
| Architecture | Unknown |