Whitespace-sensitive bang patterns (#1087, #17162)wip/whitespace-and-lookahead

This patch implements a part of GHC Proposal #229 that covers five
operators:

* the bang operator (!)
* the tilde operator (~)
* the at operator (@)
* the dollar operator ($)
* the double dollar operator ($$)

Based on surrounding whitespace, these operators are disambiguated into
bang patterns, lazy patterns, strictness annotations, type
applications, splices, and typed splices.

This patch doesn't cover the (-) operator or the -Woperator-whitespace
warning, which are left as future work.

1 files changed, 21 insertions, 0 deletions

diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs
index b1ba7bf4b2..de7ec7ec81 100644
--- a/compiler/prelude/TysWiredIn.hs
+++ b/compiler/prelude/TysWiredIn.hs
@@ -260,6 +260,27 @@ eqTyConName   = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "~")   eqTyConK
 eqDataConName = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "Eq#") eqDataConKey eqDataCon
 eqSCSelIdName = mkWiredInIdName gHC_TYPES (fsLit "eq_sel") eqSCSelIdKey eqSCSelId
 
+{- Note [eqTyCon (~) is built-in syntax]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The (~) type operator used in equality constraints (a~b) is considered built-in
+syntax. This has a few consequences:
+
+* The user is not allowed to define their own type constructors with this name:
+
+    ghci> class a ~ b
+    <interactive>:1:1: error: Illegal binding of built-in syntax: ~
+
+* Writing (a ~ b) does not require enabling -XTypeOperators. It does, however,
+  require -XGADTs or -XTypeFamilies.
+
+* The (~) type operator is always in scope. It doesn't need to be be imported,
+  and it cannot be hidden.
+
+* We have a bunch of special cases in the compiler to arrange all of the above.
+
+There's no particular reason for (~) to be special, but fixing this would be a
+breaking change.
+-}
 eqTyCon_RDR :: RdrName
 eqTyCon_RDR = nameRdrName eqTyConName