Commit 2d6d907d authored by's avatar

Comments (only) in TcFlatten

parent 1934f7f1
......@@ -797,19 +797,26 @@ flattenManyNom ev tys
{- Note [Flattening]
flatten ty ==> (xi, cc)
flatten ty ==> (xi, co)
xi has no type functions, unless they appear under ForAlls
cc = Auxiliary given (equality) constraints constraining
the fresh type variables in xi. Evidence for these
is always the identity coercion, because internally the
fresh flattening skolem variables are actually identified
with the types they have been generated to stand in for.
co :: xi ~ ty
Note that it is flatten's job to flatten *every type function it sees*.
flatten is only called on *arguments* to type functions, by canEqGiven.
Flattening also:
* zonks, removing any metavariables, and
* applies the substitution embodied in the inert set
Because flattening zonks and the returned coercion ("co" above) is also
zonked, it's possible that (co :: xi ~ ty) isn't quite true, as ty (the
input to the flattener) might not be zonked. After zonking everything,
(co :: xi ~ ty) will be true, however. It is for this reason that we
occasionally have to explicitly zonk, when (co :: xi ~ ty) is important
even before we zonk the whole program. (In particular, this is why the
zonk in flatten_tyvar3 is necessary.)
Flattening a type also means flattening its kind. In the case of a type
variable whose kind mentions a type family, this might mean that the result
of flattening has a cast in it.
......@@ -1349,6 +1356,7 @@ flatten_tyvar3 tv
-- This zonk is necessary because we might later see the tv's kind
-- in canEqTyVarTyVar (where we use getCastedTyVar_maybe).
-- If you remove it, then e.g. dependent/should_fail/T11407 panics
-- See also Note [Flattening]
; return (FTRCasted (setTyVarKind tv orig_kind) kind_co) }
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