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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2019-04-30 11:28:41 -0400 |
|---|---|---|
| committer | Ömer Sinan Ağacan <omeragacan@gmail.com> | 2019-05-03 21:54:50 +0300 |
| commit | cc495d5777c01ef62129df15caacf87b0e430c6b (patch) | |
| tree | 98367d77415752a0b21e0bcb9a5cacd233de32c5 /compiler/iface | |
| parent | 87bc954ab65aaf08b4f59cf46bd2916acd69ea73 (diff) | |
| download | haskell-cc495d5777c01ef62129df15caacf87b0e430c6b.tar.gz | |
Make equality constraints in kinds invisible
Issues #12102 and #15872 revealed something strange about the way GHC
handles equality constraints in kinds: it treats them as _visible_
arguments! This causes a litany of strange effects, from strange
error messages
(https://gitlab.haskell.org/ghc/ghc/issues/12102#note_169035)
to bizarre `Eq#`-related things leaking through to GHCi output, even
without any special flags enabled.
This patch is an attempt to contain some of this strangeness.
In particular:
* In `TcHsType.etaExpandAlgTyCon`, we propagate through the
`AnonArgFlag`s of any `Anon` binders. Previously, we were always
hard-coding them to `VisArg`, which meant that invisible binders
(like those whose kinds were equality constraint) would mistakenly
get flagged as visible.
* In `ToIface.toIfaceAppArgsX`, we previously assumed that the
argument to a `FunTy` always corresponding to a `Required`
argument. We now dispatch on the `FunTy`'s `AnonArgFlag` and map
`VisArg` to `Required` and `InvisArg` to `Inferred`. As a
consequence, the iface pretty-printer correctly recognizes that
equality coercions are inferred arguments, and as a result,
only displays them in `-fprint-explicit-kinds` is enabled.
* Speaking of iface pretty-printing, `Anon InvisArg` binders were
previously being pretty-printed like `T (a :: b ~ c)`, as if they
were required. This seemed inconsistent with other invisible
arguments (that are printed like `T @{d}`), so I decided to switch
this to `T @{a :: b ~ c}`.
Along the way, I also cleaned up a minor inaccuracy in the users'
guide section for constraints in kinds that was spotted in
https://gitlab.haskell.org/ghc/ghc/issues/12102#note_136220.
Fixes #12102 and #15872.
Diffstat (limited to 'compiler/iface')
| -rw-r--r-- | compiler/iface/IfaceType.hs | 5 | ||||
| -rw-r--r-- | compiler/iface/ToIface.hs | 10 |
2 files changed, 11 insertions, 4 deletions
diff --git a/compiler/iface/IfaceType.hs b/compiler/iface/IfaceType.hs index 4488aef025..298e14e463 100644 --- a/compiler/iface/IfaceType.hs +++ b/compiler/iface/IfaceType.hs @@ -719,8 +719,9 @@ pprIfaceTyConBinders = sep . map go -- See Note [Pretty-printing invisible arguments] case vis of AnonTCB VisArg -> ppr_bndr True - AnonTCB InvisArg -> ppr_bndr True -- Rare; just promoted GADT data constructors - -- Should we print them differently? + AnonTCB InvisArg -> char '@' <> braces (ppr_bndr False) + -- The above case is rare. (See Note [AnonTCB InvisArg] in TyCon.) + -- Should we print these differently? NamedTCB Required -> ppr_bndr True NamedTCB Specified -> char '@' <> ppr_bndr True NamedTCB Inferred -> char '@' <> braces (ppr_bndr False) diff --git a/compiler/iface/ToIface.hs b/compiler/iface/ToIface.hs index aa4e53cfb4..535c1080f2 100644 --- a/compiler/iface/ToIface.hs +++ b/compiler/iface/ToIface.hs @@ -309,8 +309,14 @@ toIfaceAppArgsX fr kind ty_args t' = toIfaceTypeX fr t ts' = go (extendTCvSubst env tv t) res ts - go env (FunTy { ft_res = res }) (t:ts) -- No type-class args in tycon apps - = IA_Arg (toIfaceTypeX fr t) Required (go env res ts) + go env (FunTy { ft_af = af, ft_res = res }) (t:ts) + = IA_Arg (toIfaceTypeX fr t) argf (go env res ts) + where + argf = case af of + VisArg -> Required + InvisArg -> Inferred + -- It's rare for a kind to have a constraint argument, but + -- it can happen. See Note [AnonTCB InvisArg] in TyCon. go env ty ts@(t1:ts1) | not (isEmptyTCvSubst env) |