Hi,
I'm running into core lint issues [1] with my determinism patch [2] and
they appear to come down to uniqAway coming up with a Unique that's already
used in the expression. That creates a collision, making next substitution
substitute more than it needs.
I have 2 questions:
1. When is it safe to use uniqAway?
2. Is my conclusion reasonable given this trace: (full trace here [3])?
I. We start out with (line 123):
CallStack (from ImplicitParams):
pprSTrace, called at compiler/coreSyn/CoreLint.hs:659:12 in ghc:CoreLint
e: EqConstr
@ (s_Xpw a_Xpz)
@ (s_Xpw b_Xpy)
@ s_Xpw
@ b_Xpy
@ a_Xpz
@~ (_N :: s_Xpw a_Xpz ~# s_Xpw a_Xpz)
@~ (_N :: s_Xpw b_Xpy ~# s_Xpw b_Xpy)
dt_aq3
fun: EqConstr
args: [TYPE: s_Xpw a_Xpz, TYPE: s_Xpw b_Xpy, TYPE: s_Xpw,
TYPE: b_Xpy, TYPE: a_Xpz, CO: _N, CO: _N,
dt_aq3]
fun_ty: forall a_apr b_aps (s_Xpw :: * -> *) b_Xpy a_Xpz.
(a_apr ~# s_Xpw a_Xpz, b_aps ~# s_Xpw b_Xpy) =>
EqTypes a_Xpz b_Xpy -> EqTypes a_apr b_aps
II. Then we create s_Xpz with uniqAway (line 499) which has the same unique
as a_Xpz above
CallStack (from ImplicitParams):
pprSTrace, called at compiler/types/TyCoRep.hs:1947:5 in ghc:TyCoRep
old_var: s_Xpy
new_var: s_Xpz
III. Which causes trouble when we want to substitute s_Xpw for s_Xpz and we
substitute a_Xpz as well (line 733):
CallStack (from ImplicitParams):
pprSTrace, called at compiler/coreSyn/CoreLint.hs:813:11 in ghc:CoreLint
substTyWith [tv] [arg_ty] body_ty forall b_XpB a_XpD.
(s_Xpw s_Xpw ~# s_Xpw a_XpD, s_Xpw
b_Xpy ~# s_Xpw b_XpB) =>
EqTypes a_XpD b_XpB -> EqTypes (s_Xpw
s_Xpw) (s_Xpw b_Xpy)
The uniqAway comes from substTyVarBndrCallback which blames to nokinds
patch.
Thanks,
Bartosz
[1] https://phabricator.haskell.org/P77
[2] https://phabricator.haskell.org/P76
[3] https://phabricator.haskell.org/P78
