"Zero-argument" lambda expressions from pretty-print strangely

An amusing corner case of the language is that you can constructor lambdas with zero arguments using Template Haskell:

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH

f :: Int
f = $(lamE [] [| 42 |])

But if you try to compile that with -ddump-splices on, it'll look quite funny:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs -ddump-splices
GHCi, version 8.2.0.20170616: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )
Bug.hs:6:7-22: Splicing expression lamE [] [| 42 |] ======> \ -> 42
Ok, modules loaded: Main.

Oh dear, \ -> 42 isn't a valid expression at all. The same thing happens with the Template Haskell pretty-printer:

λ> import Language.Haskell.TH
λ> :set -XTemplateHaskell
λ> putStrLn $(lamE [] [| 42 |] >>= stringE . pprint)
<interactive>:4:12-48: Splicing expression
    lamE [] [| 42 |] >>= stringE . pprint ======> "\ -> 42"
\ -> 42

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug TemplateHaskell Trac import labels

Trac metadata

Trac field	Value
Differential revisions	- → D3664

mentioned in commit 3c4537ea

closed

changed milestone to %8.4.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Pnormal label