User-defined operators: Documentation

docs/contracts/using-for.rst

@@ -1,4 +1,4 @@
-.. index:: ! using for, library
+.. index:: ! using for, library, ! operator; user-defined
 
 .. _using-for:
 
@@ -6,37 +6,87 @@
 Using For
 *********
 
-The directive ``using A for B;`` can be used to attach
-functions (``A``) as member functions to any type (``B``).
-These functions will receive the object they are called on
+The directive ``using A for B`` can be used to attach
+functions (``A``) as operators to user-defined value types
+or as member functions to any type (``B``).
+The member functions receive the object they are called on
 as their first parameter (like the ``self`` variable in Python).
+The operator functions receive operands as parameters.
 
 It is valid either at file level or inside a contract,
 at contract level.
 
 The first part, ``A``, can be one of:
 
-- A list of file-level or library functions (e.g. ``using {f, g, h, L.t} for uint;``) -
-  only those functions will be attached to the type as member functions.
-  Note that private library functions can only be specified when ``using for`` is inside the library.
-- The name of a library (e.g. ``using L for uint;``) -
-  all non-private functions of the library are attached to the type.
+- A list of functions, optionally with an operator name assigned (e.g.
+  ``using {f, g as +, h, L.t} for uint``).
+  If no operator is specified, the function can be either a library function or a free function and
+  is attached to the type as a member function.
+  Otherwise it must be a free function and it becomes the definition of that operator on the type.
+- The name of a library (e.g. ``using L for uint``) -
+  all non-private functions of the library are attached to the type
+  as member functions
 
 At file level, the second part, ``B``, has to be an explicit type (without data location specifier).
 Inside contracts, you can also use ``*`` in place of the type (e.g. ``using L for *;``),
 which has the effect that all functions of the library ``L``
 are attached to *all* types.
 
-If you specify a library, *all* functions in the library get attached,
+If you specify a library, *all* non-private functions in the library get attached,
 even those where the type of the first parameter does not
 match the type of the object. The type is checked at the
 point the function is called and function overload
 resolution is performed.
 
-If you use a list of functions (e.g. ``using {f, g, h, L.t} for uint;``),
+If you use a list of functions (e.g. ``using {f, g, h, L.t} for uint``),
 then the type (``uint``) has to be implicitly convertible to the
 first parameter of each of these functions. This check is
 performed even if none of these functions are called.
+Note that private library functions can only be specified when ``using for`` is inside a library.
+
+If you define an operator (e.g. ``using {f as +} for T``), then the type (``T``) must be a
+:ref:`user-defined value type <user-defined-value-types>` and the definition must be a ``pure`` function.
+Operator definitions must be global.
+The following operators can be defined this way:
+
++------------+----------+---------------------------------------------+
+| Category   | Operator | Possible signatures                         |
++============+==========+=============================================+
+| Bitwise    | ``&``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``|``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``^``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``~``    | ``function (T) pure returns (T)``           |
++------------+----------+---------------------------------------------+
+| Arithmetic | ``+``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``-``    | ``function (T, T) pure returns (T)``        |
+|            +          +---------------------------------------------+
+|            |          | ``function (T) pure returns (T)``           |
+|            +----------+---------------------------------------------+
+|            | ``*``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``/``    | ``function (T, T) pure returns (T)``        |
+|            +----------+---------------------------------------------+
+|            | ``%``    | ``function (T, T) pure returns (T)``        |
++------------+----------+---------------------------------------------+
+| Comparison | ``==``   | ``function (T, T) pure returns (bool)``     |
+|            +----------+---------------------------------------------+
+|            | ``!=``   | ``function (T, T) pure returns (bool)``     |
+|            +----------+---------------------------------------------+
+|            | ``<``    | ``function (T, T) pure returns (bool)``     |
+|            +----------+---------------------------------------------+
+|            | ``<=``   | ``function (T, T) pure returns (bool)``     |
+|            +----------+---------------------------------------------+
+|            | ``>``    | ``function (T, T) pure returns (bool)``     |
+|            +----------+---------------------------------------------+
+|            | ``>=``   | ``function (T, T) pure returns (bool)``     |
++------------+----------+---------------------------------------------+
+
+Note that unary and binary ``-`` need separate definitions.
+The compiler will choose the right definition based on how the operator is invoked.
 
 The ``using A for B;`` directive is active only within the current
 scope (either the contract or the current module/source unit),
@@ -46,7 +96,7 @@ outside of the contract or module in which it is used.
 When the directive is used at file level and applied to a
 user-defined type which was defined at file level in the same file,
 the word ``global`` can be added at the end. This will have the
-effect that the functions are attached to the type everywhere
+effect that the functions and operators are attached to the type everywhere
 the type is available (including other files), not only in the
 scope of the using statement.
 
@@ -150,3 +200,37 @@ if you pass memory or value types, a copy will be performed, even in case of the
 ``self`` variable. The only situation where no copy will be performed
 is when storage reference variables are used or when internal library
 functions are called.
+
+Another example shows how to define a custom operator for a user-defined type:
+
+.. code-block:: solidity
+
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity ^0.8.19;
+
+    type UFixed16x2 is uint16;
+
+    using {
+        add as +,
+        div as /
+    } for UFixed16x2 global;
+
+    uint32 constant SCALE = 100;
+
+    function add(UFixed16x2 a, UFixed16x2 b) pure returns (UFixed16x2) {
+        return UFixed16x2.wrap(UFixed16x2.unwrap(a) + UFixed16x2.unwrap(b));
+    }
+
+    function div(UFixed16x2 a, UFixed16x2 b) pure returns (UFixed16x2) {
+        uint32 a32 = UFixed16x2.unwrap(a);
+        uint32 b32 = UFixed16x2.unwrap(b);
+        uint32 result32 = a32 * SCALE / b32;
+        require(result32 <= type(uint16).max, "Divide overflow");
+        return UFixed16x2.wrap(uint16(a32 * SCALE / b32));
+    }
+
+    contract Math {
+        function avg(UFixed16x2 a, UFixed16x2 b) public pure returns (UFixed16x2) {
+            return (a + b) / UFixed16x2.wrap(200);
+        }
+    }