Fix pretty-printing of zero-argument lambda expressions

Using Template Haskell, one can construct lambda expressions with no
arguments. The pretty-printer isn't aware of this fact, however. This
changes that.

Test Plan: make test TEST=T13856

Reviewers: bgamari, austin, goldfire

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13856

Differential Revision: https://phabricator.haskell.org/D3664

compiler/hsSyn/Convert.hs

@@ -784,6 +784,10 @@ cvtl e = wrapL (cvt e)
                             ; t' <- cvtType t
                             ; tp <- wrap_apps t'
                             ; return $ HsAppType e' $ mkHsWildCardBndrs tp }
+    cvt (LamE [] e)    = cvt e -- Degenerate case. We convert the body as its
+                               -- own expression to avoid pretty-printing
+                               -- oddities that can result from zero-argument
+                               -- lambda expressions. See #13856.
     cvt (LamE ps e)    = do { ps' <- cvtPats ps; e' <- cvtl e
                             ; return $ HsLam (mkMatchGroup FromSource
                                              [mkSimpleMatch LambdaExpr ps' e'])}

libraries/template-haskell/Language/Haskell/TH/Ppr.hs

@@ -143,6 +143,7 @@ pprExp i (InfixE (Just e1) op (Just e2))
 pprExp _ (InfixE me1 op me2) = parens $ pprMaybeExp noPrec me1
                                     <+> pprInfixExp op
                                     <+> pprMaybeExp noPrec me2
+pprExp i (LamE [] e) = pprExp i e -- #13856
 pprExp i (LamE ps e) = parensIf (i > noPrec) $ char '\\' <> hsep (map (pprPat appPrec) ps)
                                            <+> text "->" <+> ppr e
 pprExp i (LamCaseE ms) = parensIf (i > noPrec)

testsuite/tests/th/T13856.hs

+{-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_GHC -ddump-splices #-}
+module T13856 where
+
+import Language.Haskell.TH
+
+f :: Int
+f = $(lamE [] [| 42 |])

testsuite/tests/th/T13856.stderr

+T13856.hs:8:7-22: Splicing expression lamE [] [| 42 |] ======> 42

testsuite/tests/th/all.T

@@ -388,3 +388,4 @@ test('T13618', normal, compile_and_run, ['-v0'])
 test('T13642', normal, compile_fail, ['-v0'])
 test('T13781', normal, compile, ['-v0'])
 test('T13782', normal, compile, [''])
+test('T13856', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])