Bizarre pretty-printing of closed type families in GHCi

Load this code:

{-# LANGUAGE TypeFamilies #-}
module Bug where

type family F a where
  F [Int] = Bool
  F [a]   = Double
  F (a b) = Char

into GHCi 8.0.1, 8.0.2, or HEAD, and run :i F. You'll see this:

$ /opt/ghc/8.0.2/bin/ghci Bug.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, modules loaded: Bug.
λ> :i F
type family F a :: *
  where
      F [Int] = Bool
    [a] F [a] = Double
    [b, (a :: * -> *)] F (a b) = Char
        -- Defined at Bug.hs:4:1

Compare this with GHCi 7.10.3, where the pretty-printing is far more sane:

$ /opt/ghc/7.10.3/bin/ghci Bug.hs
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, modules loaded: Bug.
λ> :i F
type family F a :: * where
    F [Int] = Bool
    F [a] = Double
    F (a b) = Char
        -- Defined at Bug.hs:4:1

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added GHCi Tbug Trac import labels

It looks like 67465497 (Kind equalities) is the culprit.

In particular, before that commit, you had to enable -fprint-explicit-foralls in order to see something like this. In GHCi 7.10.3:

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Bug              ( ../Bug.hs, interpreted )
Ok, modules loaded: Bug.
λ> :set -fprint-explicit-foralls 
λ> :i F
type family F a :: * where
    F [Int] = Bool
  forall a. F [a] = Double
  forall (a :: * -> *) b. F (a b) = Char
        -- Defined at ../Bug.hs:4:1

But in GHC 8.0.1 and later, -fprint-explicit-foralls has no effect on the output. This part of the commit is responsible:

diff --git a/compiler/iface/IfaceSyn.hs b/compiler/iface/IfaceSyn.hs
index 307a448..247566c 100644 (file)
--- a/compiler/iface/IfaceSyn.hs
+++ b/compiler/iface/IfaceSyn.hs
@@ -511,14 +511,20 @@ pprAxBranch :: SDoc -> IfaceAxBranch -> SDoc
 -- be a branch for an imported TyCon, so it would be an ExtName
 -- So it's easier to take an SDoc here
 pprAxBranch pp_tc (IfaceAxBranch { ifaxbTyVars = tvs
-                                  , ifaxbLHS = pat_tys
-                                  , ifaxbRHS = rhs
-                                  , ifaxbIncomps = incomps })
-  = hang (pprUserIfaceForAll tvs)
-       2 (hang pp_lhs 2 (equals <+> ppr rhs))
+                                 , ifaxbCoVars = cvs
+                                 , ifaxbLHS = pat_tys
+                                 , ifaxbRHS = rhs
+                                 , ifaxbIncomps = incomps })
+  = hang ppr_binders 2 (hang pp_lhs 2 (equals <+> ppr rhs))
     $+$
     nest 2 maybe_incomps
   where
+    ppr_binders
+      | null tvs && null cvs = empty
+      | null cvs             = brackets (pprWithCommas pprIfaceTvBndr tvs)
+      | otherwise
+      = brackets (pprWithCommas pprIfaceTvBndr tvs <> semi <+>
+                  pprWithCommas pprIfaceIdBndr cvs)
     pp_lhs = hang pp_tc 2 (pprParendIfaceTcArgs pat_tys)
     maybe_incomps = ppUnless (null incomps) $ parens $
                     ptext (sLit "incompatible indices:") <+> ppr incomps

Notice that we're not using pprUserIfaceForAll anymore (which consults -fprint-explicit-foralls), but instead we're using pprWithCommas, which causes the strange list-like output that we see today.

Richard, do you have a strong opinion on how closed type families should be rendered? My inclination would be to revert back to the old behavior, but maybe you had a reason for choosing this strategy.

I do remember that this change was intentional (that is, I'm not surprised by your report) but the reasons are cloudy. I'd be happy if the binders were printed only with -fprint-explicit-foralls and agree that doing so is an improvement.

changed milestone to %8.4.1

Trac metadata

Trac field	Value
Differential revisions	- → D3497

mentioned in commit da792e47

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added TypeFamilies label

added Pnormal label