Improve NegativeLiterals (#18022, GHC Proposal #344)

Before this patch, NegativeLiterals used to parse x-1 as x (-1).

This may not be what the user expects, and now it is fixed:
x-1 is parsed as (-) x 1.

We achieve this by the following requirement:

  * When lexing a negative literal,
    it must not be preceded by a 'closing token'.

This also applies to unboxed literals, e.g. -1#.

See GHC Proposal #229 for the definition of a closing token.

A nice consequence of this change is that -XNegativeLiterals becomes a
subset of -XLexicalNegation. In other words, enabling both of those
extensions has the same effect as enabling -XLexicalNegation alone.

4 files changed, 98 insertions, 18 deletions

diff --git a/testsuite/tests/parser/should_compile/LexNegVsNegLit.hs b/testsuite/tests/parser/should_compile/LexNegVsNegLit.hs
deleted file mode 100644
index 665893e95b..0000000000
--- a/testsuite/tests/parser/should_compile/LexNegVsNegLit.hs
+++ /dev/null
@@ -1,17 +0,0 @@
-{-# LANGUAGE NegativeLiterals, LexicalNegation #-}
-
-module LexNegVsNegLit where
-
--- NegativeLiterals specifies that we parse x-1 as x (-1), even though it's
--- considered a shortcoming.
---
--- LexicalNegation does not change that.
---
-b :: Bool
-b = even-1  -- parsed as: even (-1)
-            -- so it is well-typed.
-            --
-            -- with LexicalNegation alone, we'd get (-) even 1,
-            -- but NegativeLiterals takes precedence here.
-
--- See also: GHC Proposal #344
diff --git a/testsuite/tests/parser/should_compile/NegativeLiterals.hs b/testsuite/tests/parser/should_compile/NegativeLiterals.hs
new file mode 100644
index 0000000000..62d14ab678
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/NegativeLiterals.hs
@@ -0,0 +1,57 @@
+{-# LANGUAGE NegativeLiterals, MagicHash, BinaryLiterals #-}
+
+module NegativeLiterals where
+
+import GHC.Exts
+
+------------------------------------
+-- Prefix occurrence of the minus --
+------------------------------------
+
+p1 :: Bool
+p1 = even -2     -- parsed as:  even (-2)
+
+p2 :: Int
+p2 = I# -1#      -- parsed as:  I# (-1#)
+
+p3 :: Int
+p3 = floor -2.4  -- parsed as:  floor (-2.4)
+
+p4 :: Float
+p4 = F# -0.01#   -- parsed as:  F# (-0.01#)
+
+p5 :: Double
+p5 = D# -0.01##  -- parsed as:  D# (-0.01##)
+
+p6 :: Bool
+p6 =   even -0b10  -- parsed as: even (-2)
+    || even -0o10  -- parsed as: even (-8)
+    || even -0x10  -- parsed as: even (-16)
+
+-----------------------------------------
+-- Tight infix occurrence of the minus --
+-----------------------------------------
+
+ti1 :: Integer -> Integer
+ti1 x = x-2       -- parsed as:  (-) x 1
+
+ti2 :: Int# -> Int#
+ti2 x = x-1#      -- parsed as:  (-) x 1#
+  where (-) = (-#)
+
+ti3 :: Double -> Double
+ti3 x = x-2.4     -- parsed as:  (-) x 2.4
+
+ti4 :: Float# -> Float#
+ti4 x = x-0.1#    -- parsed as:  (-) x 0.1#
+  where (-) = minusFloat#
+
+ti5 :: Double# -> Double#
+ti5 x = x-0.1##   -- parsed as:  (-) x 0.1##
+  where (-) = (-##)
+
+ti6 :: Integer -> [Integer]
+ti6 x =
+  [ x-0b10,    -- parsed as: (-) x 2
+    x-0o10,    -- parsed as: (-) x 8
+    x-0x10  ]  -- parsed as: (-) x 16
diff --git a/testsuite/tests/parser/should_compile/NegativeLiteralsNoExt.hs b/testsuite/tests/parser/should_compile/NegativeLiteralsNoExt.hs
new file mode 100644
index 0000000000..9c23514d7d
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/NegativeLiteralsNoExt.hs
@@ -0,0 +1,39 @@
+{-# LANGUAGE NoNegativeLiterals, MagicHash, BinaryLiterals #-}
+
+-- Even when NegativeLiterals are disabled,
+-- we parse unboxed literals appropriately.
+module NegativeLiteralsNoExt where
+
+import GHC.Exts
+
+------------------------------------
+-- Prefix occurrence of the minus --
+------------------------------------
+
+p2 :: Int
+p2 = I# -1#      -- parsed as:  I# (-1#)
+
+p4 :: Float
+p4 = F# -0.01#   -- parsed as:  F# (-0.01#)
+
+p5 :: Double
+p5 = D# -0.01##  -- parsed as:  D# (-0.01##)
+
+-----------------------------------------
+-- Tight infix occurrence of the minus --
+-----------------------------------------
+
+ti2 :: Int# -> Int#
+ti2 x = x-1#      -- parsed as:  (-) x 1#
+  where (-) = (-#)
+
+ti3 :: Double -> Double
+ti3 x = x-2.4     -- parsed as:  (-) x 2.4
+
+ti4 :: Float# -> Float#
+ti4 x = x-0.1#    -- parsed as:  (-) x 0.1#
+  where (-) = minusFloat#
+
+ti5 :: Double# -> Double#
+ti5 x = x-0.1##   -- parsed as:  (-) x 0.1##
+  where (-) = (-##)
diff --git a/testsuite/tests/parser/should_compile/all.T b/testsuite/tests/parser/should_compile/all.T
index fb2bd6b587..48e1136daa 100644
--- a/testsuite/tests/parser/should_compile/all.T
+++ b/testsuite/tests/parser/should_compile/all.T
@@ -153,7 +153,8 @@ test('proposal-229b', normal, compile, [''])
 test('proposal-229d', normal, compile, [''])
 test('proposal-229e', normal, compile, [''])
 test('LexicalNegation', normal, compile, [''])
-test('LexNegVsNegLit', normal, compile, [''])
+test('NegativeLiterals', normal, compile, [''])
+test('NegativeLiteralsNoExt', normal, compile, [''])
 
 # We omit 'profasm' because it fails with:
 # Cannot load -prof objects when GHC is built with -dynamic