"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
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