# Expressions ## Table of contents - [Overview](#overview) - [Precedence](#precedence) - [Names](#names) - [Unqualified names](#unqualified-names) - [Qualified names and member access](#qualified-names-and-member-access) - [Operators](#operators) - [Suffix operators](#suffix-operators) - [Conversions and casts](#conversions-and-casts) - [`if` expressions](#if-expressions) - [Numeric type literal expressions](#numeric-type-literal-expressions) - [Alternatives considered](#alternatives-considered) - [References](#references) ## Overview Expressions are the portions of Carbon syntax that produce values. Because types in Carbon are values, this includes anywhere that a type is specified. ``` fn Foo(a: i32*) -> i32 { return *a; } ``` Here, the parameter type `i32*`, the return type `i32`, and the operand `*a` of the `return` statement are all expressions. ## Precedence Expressions are interpreted based on a partial [precedence ordering](https://en.wikipedia.org/wiki/Order_of_operations). Expression components which lack a relative ordering must be disambiguated by the developer, for example by adding parentheses; otherwise, the expression will be invalid due to ambiguity. Precedence orderings will only be added when it's reasonable to expect most developers to understand the precedence without parentheses. The precedence diagram is defined thusly: ```mermaid %%{init: {'themeVariables': {'fontFamily': 'monospace'}}}%% graph BT parens["(...)"] braces["{...}"] click braces "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/classes.md#literals" unqualifiedName["x"] click unqualifiedName "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/README.md#unqualified-names" top((" ")) suffixOps{"x.y x.(...) x->y x->(...) x(...) x[y]"} click suffixOps "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/README.md#suffix-operators" qualifiedType["const T partial T"] click pointer-type "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/type_operators.md" pointerType{"T*"} click pointer-type "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/type_operators.md" pointer{"*x &x"} click pointer "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/pointer.md" negation["-x"] click negation "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/arithmetic.md" complement["^x"] click complement "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/bitwise.md" incDec["++x; --x;"] click incDec "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/assignment.md" unary((" ")) as["x as T"] click as "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/implicit_conversions.md" multiplication>"x * y x / y"] click multiplication "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/arithmetic.md" addition>"x + y x - y"] click addition "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/arithmetic.md" modulo["x % y"] click modulo "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/arithmetic.md" bitwise_and>"x & y"] bitwise_or>"x | y"] bitwise_xor>"x ^ y"] click bitwise_and "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/bitwise.md" click bitwise_or "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/bitwise.md" click bitwise_xor "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/bitwise.md" shift["x << y x >> y"] click shift "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/bitwise.md" binaryOps((" ")) where["T where R"] comparison["x == y x != y x < y x <= y x > y x >= y"] click comparison "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/comparison_operators.md" not["not x"] click not "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/logical_operators.md" logicalOperand((" ")) and>"x and y"] click and "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/logical_operators.md" or>"x or y"] click or "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/logical_operators.md" logicalExpression((" ")) ref["ref x"] if>"if x then y else z"] click if "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/expressions/if.md" insideParens["(...)"] assignment["x = y; x $= y;"] click assignment "https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/assignment.md" expressionStatement["x;"] top --> parens & braces & unqualifiedName suffixOps --> top qualifiedType --> suffixOps pointerType --> qualifiedType pointer --> suffixOps negation & complement & incDec --> pointer unary --> pointerType & negation & complement %% Use a longer arrow here to put `not` next to other unary operators not ---> suffixOps %% `as` at the same level as `where` and comparisons as -----> unary multiplication & modulo & bitwise_and & bitwise_or & bitwise_xor & shift --> unary addition --> multiplication binaryOps --> addition & modulo & bitwise_and & bitwise_or & bitwise_xor & shift where --> binaryOps comparison --> binaryOps logicalOperand --> comparison & not %% This helps group `and` and `or` together classDef hidden display: none; HIDDEN:::hidden ~~~ logicalOperand and & or --> logicalOperand logicalExpression --> as & where & and & or ref & expressionStatement --> logicalExpression if ---> ref insideParens & assignment --> if ``` The diagram's attributes are: - Each non-empty node represents a precedence group. Empty circles are used to simplify the graph, and do not represent a precedence group. - When an expression is composed from different precedence groups, the interpretation is determined by the precedence edges: - A precedence edge A --> B means that A is lower precedence than B, so A can contain B without parentheses. For example, `or --> not` means that `not x or y` is treated as `(not x) or y`. - Precedence edges are transitive. For example, `or --> == --> as` means that `or` is lower precedence than `as`. - When a binary operator expression is composed from a single precedence group, the interpretation is determined by the [associativity](https://en.wikipedia.org/wiki/Operator_associativity) of the precedence group: ```mermaid graph TD non["Non-associative"] left>"Left associative"] ``` - For example, `+` and `-` are left-associative and in the same precedence group, so `a + b + c - d` is treated as `((a + b) + c) - d`. - Note that in Carbon, we currently only have left-associative operators. Unlike C++ and other languages, [assignment](/docs/design/assignment.md) isn't a right-associative operator, it uses its own statement. - When a unary operator expression is composed from a single precedence group, it can allow unparenthesized repetition or not: ```mermaid graph TD non["Non-repeating"] repeating{"Repeating"} ``` This is analogous to associativity for binary operators. ## Names ### Unqualified names An _unqualified name_ is a [word](../lexical_conventions/words.md) that is not a keyword and is not preceded by a period (`.`). **TODO:** Name lookup rules for unqualified names. ### Qualified names and member access A _qualified name_ is a word that appears immediately after a period or rightward arrow. Qualified names appear in the following contexts: - [Designators](/docs/design/classes.md#literals): `.` _word_ - [Simple member access expressions](member_access.md): _expression_ `.` _word_ - [Simple pointer member access expressions](member_access.md): _expression_ `->` _word_ ``` var x: auto = {.hello = 1, .world = 2}; ^^^^^ ^^^^^ qualified name ^^^^^^ ^^^^^^ designator x.hello = x.world; ^^^^^ ^^^^^ qualified name ^^^^^^^ ^^^^^^^ member access expression x.hello = (&x)->world; ^^^^^ qualified name ^^^^^^^^^^^ pointer member access expression ``` Qualified names refer to members of an entity determined by the context in which the expression appears. For a member access, the entity is named by the expression preceding the period. In a struct literal, the entity is the struct type. For example: ``` package Foo; namespace N; fn N.F() {} fn G() { // Same as `(Foo.N).F()`. // `Foo.N` names namespace `N` in package `Foo`. // `(Foo.N).F` names function `F` in namespace `N`. Foo.N.F(); } // `.n` refers to the member `n` of `{.n: i32}`. fn H(a: {.n: i32}) -> i32 { // `a.n` is resolved to the member `{.n: i32}.n`, // and names the corresponding subobject of `a`. return a.n; } fn J() { // `.n` refers to the member `n of `{.n: i32}`. H({.n = 5 as i32}); } ``` Member access expressions associate left-to-right. If the member name is more complex than a single _word_, a compound member access expression can be used, with parentheses around the member name: - _expression_ `.` `(` _expression_ `)` - _expression_ `->` `(` _expression_ `)` ``` interface I { fn F[self: Self](); } class X {} impl X as I { fn F[self: Self]() {} } // `x.I.F()` would mean `(x.I).F()`. fn Q(x: X) { x.(I.F)(); } ``` Either simple or compound member access can be part of a _pointer_ member access expression when an `->` is used instead of a `.`, where _expression_ `->` _..._ is syntactic sugar for `(` `*` _expression_ `)` `.` _..._. ## Operators Most expressions are modeled as operators: | Category | Operator | Syntax | Function | | ---------- | ----------------------------------- | --------- | --------------------------------------------------------------------- | | Call | `()` (unary) | `x(...)` | Function call: the value returned by calling the function `x`. | | Call | [`[]`](indexing.md) (unary) | `x[y]` | Subscripting or indexing: returns the element `y` of `x`. | | Pointer | [`*`](pointer_operators.md) (unary) | `*x` | Pointer dereference: the object pointed to by `x`. | | Pointer | [`&`](pointer_operators.md) (unary) | `&x` | Address-of: a pointer to the object `x`. | | Arithmetic | [`-`](arithmetic.md) (unary) | `-x` | The negation of `x`. | | Bitwise | [`^`](bitwise.md) (unary) | `^x` | The bitwise complement of `x`. | | Arithmetic | [`+`](arithmetic.md) | `x + y` | The sum of `x` and `y`. | | Arithmetic | [`-`](arithmetic.md) (binary) | `x - y` | The difference of `x` and `y`. | | Arithmetic | [`*`](arithmetic.md) | `x * y` | The product of `x` and `y`. | | Arithmetic | [`/`](arithmetic.md) | `x / y` | `x` divided by `y`, or the quotient thereof. | | Arithmetic | [`%`](arithmetic.md) | `x % y` | `x` modulo `y`. | | Bitwise | [`&`](bitwise.md) | `x & y` | The bitwise AND of `x` and `y`. | | Bitwise | [`\|`](bitwise.md) | `x \| y` | The bitwise OR of `x` and `y`. | | Bitwise | [`^`](bitwise.md) (binary) | `x ^ y` | The bitwise XOR of `x` and `y`. | | Bitwise | [`<<`](bitwise.md) | `x << y` | `x` bit-shifted left `y` places. | | Bitwise | [`>>`](bitwise.md) | `x >> y` | `x` bit-shifted right `y` places. | | Conversion | [`as`](as_expressions.md) | `x as T` | Converts the value `x` to the type `T`. | | Comparison | [`==`](comparison_operators.md) | `x == y` | Equality: `true` if `x` is equal to `y`. | | Comparison | [`!=`](comparison_operators.md) | `x != y` | Inequality: `true` if `x` is not equal to `y`. | | Comparison | [`<`](comparison_operators.md) | `x < y` | Less than: `true` if `x` is less than `y`. | | Comparison | [`<=`](comparison_operators.md) | `x <= y` | Less than or equal: `true` if `x` is less than or equal to `y`. | | Comparison | [`>`](comparison_operators.md) | `x > y` | Greater than: `true` if `x` is greater than to `y`. | | Comparison | [`>=`](comparison_operators.md) | `x >= y` | Greater than or equal: `true` if `x` is greater than or equal to `y`. | | Logical | [`and`](logical_operators.md) | `x and y` | A short-circuiting logical AND: `true` if both operands are `true`. | | Logical | [`or`](logical_operators.md) | `x or y` | A short-circuiting logical OR: `true` if either operand is `true`. | | Logical | [`not`](logical_operators.md) | `not x` | Logical NOT: `true` if the operand is `false`. | The binary arithmetic and bitwise operators also have [compound assignment](/docs/design/assignment.md) forms. These are statements rather than expressions, and do not produce a value. ## Suffix operators These operators act like unary postfix operators for purposes of precedence: - [Member access operators](member_access.md), like `x.y` and the dereferencing variant `x->y`, only have an expression on their left-hand side. The right-hand side is a name. - The [compound member access operators](member_access.md), `x.(...)` and `x->(...)`, have an expression as their second operand, but put that expression in parentheses and so it doesn't participate in the precedence considerations of its first operand. - The [indexing operator](indexing.md), `x[y]`, similarly puts its second operand in matching square brackets. - The call operator, `x(...)`, takes a comma-separated list of arguments, but again puts them in parentheses that clearly separate them for precedence purposes. ## Conversions and casts When an expression appears in a context in which an expression of a specific type is expected, [implicit conversions](implicit_conversions.md) are applied to convert the expression to the target type. Expressions can also be converted to a specific type using an [`as` expression](as_expressions.md). ``` fn Bar(n: i32); fn Baz(n: i64) { // OK, same as Bar(n as i32) Bar(n); } ``` ## `if` expressions An [`if` expression](if.md) chooses between two expressions. ``` fn Run(args: Span(StringView)) { var file: StringView = if args.size() > 1 then args[1] else "/dev/stdin"; } ``` `if` expressions are analogous to `?:` ternary expressions in C and C++. ## Numeric type literal expressions Carbon's syntax provides a simple way to represent different types of integers and floating-point numbers. Each type is identified with a keyword-like syntax, prefixed with either `i`, `u`, or `f` followed by a multiple of 8, representing the size in bits of the data type. These are referred to as [numeric type literals](literals.md#numeric-type-literals). ## Alternatives considered Other expression documents will list more alternatives; this lists alternatives not noted elsewhere. - [Total order](/proposals/p0555.md#total-order) - [Different precedence for different operands](/proposals/p0555.md#different-precedence-for-different-operands) - [Require less than a partial order](/proposals/p0555.md#require-less-than-a-partial-order) ## References Other expression documents will list more references; this lists references not noted elsewhere. - Proposal [#555: Operator precedence](https://github.com/carbon-language/carbon-lang/pull/555).