Ghc 40798

message-index/messages/GHC-40798/example1/after/Main.hs

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+{-# OPTIONS -Woperator-whitespace #-}
+
+module Main where
+
+main = print "notice the usage of whitespace below"
+
+f a b = a + b

message-index/messages/GHC-40798/example1/before/Main.hs

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+{-# OPTIONS -Woperator-whitespace #-}
+
+module Main where
+
+main = print "notice the usage of whitespace below"
+
+f a b = a+ b -- this one is a warning
+
+g a b = a +b -- this one is also a warning
+
+h a b = a+b -- this one as well

message-index/messages/GHC-40798/example1/index.md

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+---
+title: operator-whitespace
+summary: An optional warning for detecting usage of infix, suffix or prefix operators that could be parsed differently in the future due to whitespace, this warning is disabled by default.
+severity: warning
+flag: -Wall, -Wcompat, or -Woperator-whitespace
+introduced: 9.6.1
+---
+
+## Error Message
+```
+Main.hs:7:10: warning: [-Woperator-whitespace] [GHC-40798]
+    The suffix use of a ‘+’ might be repurposed as special syntax
+      by a future language extension.
+    Suggested fix: Add whitespace around ‘+’.
+  |
+7 | f a b = a+ b
+  |          ^
+
+Main.hs:9:11: warning: [-Woperator-whitespace] [GHC-40798]
+    The prefix use of a ‘+’ might be repurposed as special syntax
+      by a future language extension.
+    Suggested fix: Add whitespace around ‘+’.
+  |
+9 | g a b = a +b
+  |           ^
+
+Main.hs:11:10: warning: [-Woperator-whitespace] [GHC-40798]
+    The tight infix use of a ‘+’ might be repurposed as special syntax
+      by a future language extension.
+    Suggested fix: Add whitespace around ‘+’.
+   |
+11 | h a b = a+b
+   |          ^
+
+```

message-index/messages/GHC-40798/index.md

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+---
+title: Operator whitespace
+summary: An optional warning for detecting usage of infix, suffix or prefix operators that could be parsed differently in future due to whitespace.
+severity: warning
+introduced: 9.6.1
+---
+
+GHC distinguishes between a few different occurences for operators depending on whitespace.
+An operator can occur in the following ways:
+
+```
+a ! b   -- a loose infix occurrence
+a!b     -- a tight infix occurrence
+a !b    -- a prefix occurrence
+a! b    -- a suffix occurrence
+```
+A loose infix occurrence will always be treated as an operator wheres any of the remaining cases can be redefined by some future language extension.
+
+Consider `-XBangPatterns`
+
+```
+a ! b = <rhs>  -- Infix (!)
+f !a = <rhs>   -- Bang pattern
+```
+
+the above code is possible today, whereas
+
+```
+a +b = <rhs>
+```
+
+would break if a language extension that introduces a prefix occurence for plus is introduced.
+
+The documentation for the `-Woperator-whitespace` flag can be found in the [GHC user guide](https://downloads.haskell.org/ghc/latest/docs/html/users_guide/using-warnings.html?highlight=whitespace#ghc-flag--Woperator-whitespace)