Poor pretty-printing of GADT constructors in GHCi
I recently loaded this file into GHCi:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where
import Data.Proxy
data Foo1 (a :: k) where
MkFoo1a :: Proxy a -> Int -> Foo1 a
MkFoo1b :: { a :: Proxy a, b :: Int } -> Foo1 a
data family Foo2 (a :: k)
data instance Foo2 (a :: k) where
MkFoo2a :: Proxy a -> Int -> Foo2 a
MkFoo2b :: { c :: Proxy a, d :: Int} -> Foo2 a
And when I queried these datatypes with :info
, I saw this:
$ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs -fprint-explicit-kinds
GHCi, version 8.7.20181129: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Ok, one module loaded.
λ> :i Foo1 Foo2
type role Foo1 nominal phantom
data Foo1 @k (a :: k) where
MkFoo1a :: forall k (a :: k). (Proxy @{k} a) -> Int -> Foo1 k a
MkFoo1b :: forall k (a :: k).
{a :: Proxy @{k} a, b :: Int} -> Foo1 k a
-- Defined at Bug.hs:8:1
data family Foo2 @k (a :: k) -- Defined at Bug.hs:12:1
data instance forall k (a :: k). Foo2 @k a where
MkFoo2a :: forall k (a :: k). (Proxy @{k} a) -> Int -> Foo2 @k a
MkFoo2b :: forall k (a :: k).
{c :: Proxy @{k} a, d :: Int} -> Foo2 @k a
-- Defined at Bug.hs:13:15
Yuck. A couple of icky things to note here:
- For some reason, the
Proxy @{k} a
field is needlessly parenthesized inMkFoo1a
andMkFoo2a
. This does //not// happen when record syntax is used, as demonstrated withMkFoo1b
andMkFoo2b
. - Even more strangely, despite the fact that
k
is a specified argument, we're pretty-printing the return type ofMkFoo1{a,b}
asFoo k a
, notFoo @k a
. This problem doesn't appear to happen for data family instances, sinceMkFoo2{a,b}
don't have this issue.
Patch incoming.
Trac metadata
Trac field | Value |
---|---|
Version | 8.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |