#8356: Strangeness with FunDeps ----------------------------------------------+---------------------------- Reporter: ksf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by rwbarton): The error makes sense to me: `sy`, `a`, `b` and `((sy ::: a) ': xs)` do not jointly determine `((sy ::: b) ': ys)`, because the whole `ys` part is free. (And the same for the other fundep.) Perhaps the error message could be more specific, saying what I said above? something like {{{ Illegal instance declaration for [...] Multiple uses of this instance may be inconsistent with the functional dependencies of the class: `sy', `a', `b' and `((sy ::: a) ': xs)' do not jointly determine `((sy ::: b) ': ys)' }}} Extra clarity would be good here since there are existing libraries that use fundeps incorrectly, that will stop compiling in 7.8, and I anticipate a slew of bug reports otherwise. See for example http://ghc.haskell.org/trac/ghc/ticket/1241#comment:22.

What I'm trying to do is to express "xs is ys and ys is xs with a and b interchanged at sy", all in a single predicate because my current type family implementation needs two and explodes the inferred types.

In that case, perhaps you intended {{{ instance Replaced sy a b ((sy ::: a) ': xs) ((sy ::: b) ': xs) -- note xs twice }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8356#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8356: Strangeness with FunDeps ----------------------------------------------+---------------------------- Reporter: ksf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by ksf): For reference, here's the whole code: http://lpaste.net/93383 ...trying to minimise the inferred type for {{{foo}}}, both with and without {{{li}}} fixed. With the added {{{sy ys -> b}}} constraint, the {{{ElemTy sy ys ~ b}}} constraint on {{{mapKey}}} could be elided. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8356#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8356: Strangeness with FunDeps ----------------------------------------------+---------------------------- Reporter: ksf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by unknown ): In [changeset:bceeb0167804e3325b48d4b360fddd68e29735a2/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="bceeb0167804e3325b48d4b360fddd68e29735a2" Improve error reporting of fundep coverage condition failure This modest improvement is motivated by Trac #8356 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8356#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8356: Strangeness with FunDeps -------------------------------------------------+------------------------- Reporter: ksf | Owner: Type: bug | Status: Priority: normal | closed Component: Compiler | Milestone: Resolution: fixed | Version: 7.7 Operating System: Unknown/Multiple | Keywords: Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple typecheck/should_fail/tcfail170 | Difficulty: Blocking: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Comment (by monoidal): The testcase `tcfail170` triggers a `pprTrace` in `FunDeps.lhs`. Is this intended? {{{ $ ghc-7.7.20131027 tcfail170 [1 of 1] Compiling ShouldFail ( tcfail170.hs, tcfail170.o ) cic main:ShouldFail.C{tc roB} [[p{tv aoE} [sk]], [q{tv aoF} [sk]]] ([a{tv aoC} [tv]], [b{tv aoD} [tv]]) [(aoE, p{tv aoE} [sk])] [(aoF, q{tv aoF} [sk])] [(aoE, p{tv aoE} [sk])] [] tcfail170.hs:7:10: Illegal instance declaration for ‛C [p] [q]’ The coverage condition fails in class ‛C’ for functional dependency: ‛a -> b’ Reason: lhs type ‛[p]’ does not determine rhs type ‛[q]’ In the instance declaration for ‛C [p] [q]’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8356#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8356: Strangeness with FunDeps -------------------------------------------------+------------------------- Reporter: ksf | Owner: Type: bug | Status: Priority: normal | closed Component: Compiler | Milestone: Resolution: fixed | Version: 7.7 Operating System: Unknown/Multiple | Keywords: Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple typecheck/should_fail/tcfail170 | Difficulty: Blocking: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Comment (by Krzysztof Gogolewski ): In [changeset:"eaa4682b68348eb3b49a0bf2700abc58be820555/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="eaa4682b68348eb3b49a0bf2700abc58be820555" Remove debugging output (#8356) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8356#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler