TH eta-reduces away explicit foralls in data family instances
Consider the following code:
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where
import Language.Haskell.TH
data family F1 a b
data instance F1 [a] b = MkF1
data family F2 a
data instance F2 a = MkF2
$(do i1 <- reify ''F1
i2 <- reify ''F2
runIO $ mapM_ (putStrLn . pprint) [i1, i2]
pure [])
$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs
GHCi, version 8.7.20181101: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
data family Bug.F1 (a_0 :: *) (b_1 :: *) :: *
data instance forall (a_2 :: *). Bug.F1 ([a_2]) b_3 = Bug.MkF1
data family Bug.F2 (a_0 :: *) :: *
data instance Bug.F2 a_1 = Bug.MkF2
The output here is quite baffling:
- In the
F1
instance, we have an explicitforall
which quantifiesa_2
but notb_3
! - In the
F2
instance, there isn't an explicitforall
at all despite the fact that there ought to be one, since there is a type variablea_1
in this instance.
The culprit in both of the bullet points above is the fact that GHC eta-reduces its internal representation of data family instance axioms. This is the same thing which caused #9692 (closed) and #14179 (closed), in fact. Luckily, the same fix for those tickets will also work here.
Patch incoming.
Trac metadata
Trac field | Value |
---|---|
Version | 8.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Template Haskell |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |