carbon-lang/proposals/p0601.md

Operator tokens

Pull request

Table of contents

• Problem
• Background
• Proposal
• Details
  • Two kinds of operator tokens
  • Symbolic token list
  • Whitespace
• Rationale based on Carbon's goals
• Alternatives considered

Problem

Carbon needs a set of tokens to represent operators.

Background

Some languages have a fixed set of operator tokens. For example:

• C++ operators
  • The keyword operators and, or, etc. are lexical synonyms for corresponding symbolic operators &&, ||, etc.
• Rust operators

Other languages have extensible rules for defining operators, including the facility for a developer to define operators that aren't part of the base language. For example:

• Swift operator rules
• Haskell operator rules

Operators tokens can be formed by various rules, for example:

• At each lexing step, form the longest known operator token possible from the remaining character sequence. For example, in C++, a += b is 3 tokens and a =+ b is four tokens, because there are +, =, and += operators, but there is no =+ operator. This approach is sometimes known as "max munch".
• At each lexing step, treat the longest sequence of operator-like characters possible as an operator. The program is invalid if there is no such operator. For example, in a C++-like language using this approach, a =+ b would be invalid instead of meaning a = (+b).
• Use semantic information to determine how to split a sequence of operator characters into one or more operators, for example based on the types of the operands.

Proposal

Carbon has a fixed set of tokens that represent operators, defined by the language specification. Developers cannot define new tokens to represent new operators; there may be facilities to overload operators, but that is outside the scope of this proposal. There are two kinds of tokens that represent operators:

• Symbolic tokens consist of one or more symbol characters. In particular, such a token contains no characters that are valid in identifiers, no quote characters, and no whitespace.
• Keywords follow the lexical rules for words.

Symbolic tokens are lexed using a "max munch" rule: at each lexing step, the longest symbolic token defined by the language specification that appears starting at the current input position is lexed, if any.

Not all uses of symbolic tokens within the Carbon grammar will be as operators. For example, we will have ( and ) tokens that serve to delimit various grammar productions, and we may not want to consider . to be an operator, because its right "operand" is not an expression.

When a symbolic token is used as an operator, we use the presence or absence of whitespace around the symbolic token to determine its fixity, in the same way we expect a human reader to recognize them. For example, we want a* - 4 to treat the * as a unary operator and the - as a binary operator, while a * -4 results in the reverse. This largely requires whitespace on only one side of a unary operator and on both sides of a binary operator. However, we'd also like to support binary operators where a lack of whitespace reflects precedence such as2*x*x + 3*x + 1 where doing so is straightforward. The rules we use to achieve this are:

• There can be no whitespace between a unary operator and its operand.
• The whitespace around a binary operator must be consistent: either there is whitespace on both sides or on neither side.
• If there is whitespace on neither side of a binary operator, the token before the operator must be an identifier, a literal, or any kind of closing bracket (for example, ), ], or }), and the token after the operator must be an identifier, a literal, or any kind of opening bracket (for example, (, [, or {).

This proposal includes an initial set of symbolic tokens covering only the grammar productions that have been approved so far. This list should be extended by proposals that use additional symbolic tokens.

Details

Two kinds of operator tokens

Two kinds of operator tokens are proposed. These two kinds are intended for different uses, not as alternate spellings of the same functionality:

• Symbolic tokens are intended to be used for widely-recognized operators, such as the mathematical operators +, *, <, and so on.
  • Symbolic tokens used as operators would generally be expected to also be meaningful for some user-defined types, and should be candidates for being made overloadable once we support operator overloading.
• Keywords are intended to be used for cases such as the following:
  • Operators that perform flow control, such as and, or, throw, yield, and operators closely connected to these, such as not. It is important that these stand out from other operators as they have action that goes beyond evaluating their operands and computing a value.
  • Operators that are rare and that we do not want to spend our finite symbolic token budget on, such as perhaps xor or bit rotate.
  • Operators with very low precedence, and perhaps certain operators with very high precedence.
  • Special-purpose operators for which there is no conventional established symbol and for which we do not want to invent one, such as as.

The example operators in this section are included only to motivate the two kinds of operator token; those specific operators are not proposed as part of this proposal.

Symbolic token list

The following is the initial list of symbolic tokens recognized in a Carbon source file:

(	)	{	}	[	]
,	.	;	:	*	&
=	->	=>

This list is expected to grow over time as more symbolic tokens are required by language proposals.

Whitespace

We wish to support the use of the same symbolic token as a prefix operator, an infix operator, and a postfix operator, in some cases. In particular, we have decided in #523 that the * operator should support all three uses; this operator will be introduced in a future proposal. In order to support such usage, we want a rule that allows us to simply and unambiguously parse operators that might have all three fixities.

For example, given the expression a * - b, there are two possible parses:

• As a * (- b), multiplying a by the negation of b.
• As (a *) - b, subtracting b from the pointer type a *.

Our chosen rule to distinguish such cases is to consider the presence or absence of whitespace, as we think this strikes a good balance between simplicity and expressiveness for the programmer and simplicity and good support for error recovery in the implementation. a * -b uses the first interpretation, a* - b uses the second interpretation, and other combinations (a*-b, a *- b, a* -b, a * - b, a*- b, a *-b) are rejected as errors.

In general, we require whitespace to be present or absent around the operator to indicate its fixity, as this is a cue that a human reader would use to understand the code: binary operators have whitespace on both sides, and unary operators lack whitespace between the operator and its operand. We also make allowance for omitting the whitespace around a binary operator in cases where it aids readability to do so, such as in expressions like 2*x*x + 3*x + 1: for an operator with whitespace on neither side, if the token immediately before the operator indicates it is the end of an operand, and the token immediately after the operator indicates it is the beginning of an operand, the operator is treated as binary.

We define the set of tokens that constitutes the beginning or end of an operand as:

• Identifiers, as in x*x + y*y.
• Literals, as in 3*x + 4*y or "foo"+s.
• Brackets of any kind, facing away from the operator, as in f()*(n + 3) or args[3]*{.real=4, .imag=1}.

For error recovery purposes, this rule functions best if no expression context can be preceded by a token that looks like the end of an operand and no expression context can be followed by a token that looks like the start of an operand. One known exception to this is in function definitions:

fn F(p: Int *) -> Int * { return p; }

Both occurrences of Int * here are erroneous. The first is easy to detect and diagnose, but the second is more challenging, if {...} is a valid expression form. We expect to be able to easily distinguish between code blocks starting with { and expressions starting with { for all cases other than {}. However, the code block {} is not a reasonable body for a function with a return type, so we expect errors involving a combination of misplaced whitespace and {} to be rare, and we should be able to recover well from the remaining cases.

From the perspective of token formation, the whitespace rule means that there are four variants of each symbolic token:

• A symbolic token with whitespace on both sides is a binary variant of the token.
• A symbolic token with whitespace on neither side, where the preceding token is an identifier, literal, or closing bracket, and the following token is an identifier, literal, or (, is also a binary variant of the token.
• A symbolic token with whitespace on neither side that does not satisfy the preceding rule is a unary variant of the token.
• A symbolic token with whitespace on the left side only is a prefix variant of the token.
• A symbolic token with whitespace on the right side only is a postfix variant of the token.

When used in non-operator contexts, any variant of a symbolic token is acceptable. When used in operator contexts, only a binary variant of a token can be used as a binary operator, only a prefix or unary variant of a token can be used as a prefix operator, and only a postfix or unary variant of a token can be used as a postfix operator.

This whitespace rule has been implemented in the Carbon toolchain for all operators by tracking the presence or absence of trailing whitespace as part of a token, and in executable semantics for the * operator by forming four different token variants as described above.

The choice to disallow whitespace between a unary operator and its operand is experimental.

Rationale based on Carbon's goals

• Software and language evolution

  • By not allowing user-defined operators, we reduce the possibility that operators added to the language later will conflict with existing uses in programs. Due to the use of a max munch rule, we might add an operator that causes existing code to be interpreted differently, but such problems will be easy to detect and resolve, because we know the operator set in advance.

• Code that is easy to read, understand, and write

  • The fixed operator set means that developers don't need to understand an unbounded and extensible number of operators and precedence rules. The fixed operator set encourages functionality that does not correspond to a well-known operator symbol to be exposed by way of a named operation instead of a symbol, improving readability among developers not familiar with a codebase.
  • Requiring whitespace to be used consistently around operators reduces the possibility for confusing formatting.
  • Permitting whitespace on either both sides of a binary operator or on neither side allows expressions such as 2*x*x + 3*x + 1 to use the absence of whitespace to improve readability. Because the language officially sanctions both choices, the formatting tool can be expected to preserve the user's choice.
  • The choice to lex the longest known symbolic token rather than the longest sequence of symbolic characters makes it easier to write expressions involving a series of prefix or postfix operators, such as x = -*p;.

• Interoperability with and migration from existing C++ code

  • The fixed operator set makes a mapping between Carbon operators and C++ operators easier, by avoiding any desire to map arbitrary user-defined Carbon operators into a C++ form.
  • The choice of a fixed operator set and a "max munch" rule will be familiar to C++ developers, as it is the same approach taken by C++.
  • The whitespace rule permits the * operator to be used for all of multiplication, dereference, and pointer type formation, as in C++, while still permitting Carbon to treat type expressions as expressions.

Alternatives considered

We could lex the longest sequence of symbolic characters rather than lexing only the longest known operator.

Advantages:

• Adding new operators could be done without any change to the lexing rules.
• If unknown operators are rejected, adding new operators would carry no risk of changing the meaning of existing valid code.

Disadvantages:

• Sequences of prefix or postfix operators would require parentheses or whitespace. For example, Int** would lex as Int followed by a single ** token, and **p would lex as a single ** token followed by p, if there is no ** operator. While we could define **, ***, and so on as operators, doing so would add complexity and inconsistency to the language rules.

We could support an extensible operator set, giving the developer the option to add new operators.

Advantages:

• This would increase expressivity, especially for embedded domain-specific languages.

Disadvantages:

• This would harm readability, at least for those unfamiliar with the code using the operators.
• This could harm our ability to evolve the language, by admitting the possibility of a custom operator colliding with a newly-introduced standard operator, although this risk could be reduced by providing a separate lexical syntax for custom operators.
• We would need to either lex the longest sequence of symbolic characters we can, which has the same disadvantage discussed for that approach above, or use a more sophisticated rule to determine how to split operators -- perhaps based on what operator overloads are in scope -- increasing complexity.

We could apply different whitespace restrictions or no whitespace restrictions. See #520 for discussion of the alternatives and the leads decision.

We could require whitespace around a binary operator followed by [ or {. In particular, for examples such as:

fn F() -> Int*{ return Null; }
var n: Int = pointer_to_array^[i];

... this would allow us to form a unary operator instead of a binary operator, which is likely to be more in line with the developer's expectations.

Advantages:

• Room to add a postfix ^ dereference operator, or similarly any other postfix operator producing an array, without creating surprises for pointers to arrays.
• Allows the whitespace before the { of a function body to be consistently omitted if desired.

Disadvantages:

• The rule would be more complex, and would be asymmetric: we must allow closing square brackets before unspaced binary operators to permit things like arr[i]*3.
• Would interact badly with expression forms that begin with a [ or {, for example Time.Now()+{.seconds = 3} or names+["Lrrr"].