Crate arithmetic_parser[][src]

Expand description

Parser for arithmetic expressions with flexible definition of literals and support of type annotations.

Overall, parsed grammars are similar to Rust syntax, with a few notable differences.

Supported syntax features

  • Variables. A variable name is defined similar to Rust and other programming languages, as a sequence of alphanumeric chars and underscores that does not start with a digit.
  • Literals. The parser for literals is user-provided, thus allowing to apply the library to different domains (e.g., finite group arithmetic).
  • // and /* .. */ comments.
  • Basic arithmetic operations: +, - (binary and unary), *, /, ^ (power). The parser outputs AST with nodes organized according to the operation priority.
  • Function calls: foo(1.0, x).
  • Parentheses which predictably influence operation priority.

The parser supports both complete and streaming (incomplete) modes; the latter is useful for REPLs and similar applications.

Optional syntax features

These features can be switched on or off when defining a Parse impl by declaring the corresponding Features.

  • Tuples. A tuple is two or more elements separated by commas, such as (x, y) or (1, 2 * x). Tuples are parsed both as lvalues and rvalues.
  • Tuple destructuring. Using a tuple as an lvalue, for example, (x, y, z) = foo. The “rest” syntax is also supported, either named or unnamed: (head, ...tail) = foo, (a, ..., b, c) = foo.
  • Function definitions. A definition looks like a closure definition in Rust, e.g., |x| x - 10 or |x, y| { z = max(x, y); (z - x, z - y) }. A definition may be assigned to a variable (which is the way to define named functions).
  • Destructuring for function args. Similar to tuple destructuring, it is possible to destructure and group args in function definitions, for example, |(x, y), ...zs| { }.
  • Blocks. A block is several ;-delimited statements enclosed in {} braces, e.g, { z = max(x, y); (z - x, z - y) }. The blocks can be used in all contexts instead of a simple expression; for example, min({ z = 5; z - 1 }, 3).
  • Objects. Object is a mapping of string fields to values. Objects are defined via object expressions, which look similar to struct initialization in Rust or object initialization in JavaScript; for example, #{ x: 1, y }. (Note the # char at the start of the block; it is used to distinguish object expressions from blocks.)
  • Methods. Method call is a function call separated from the receiver with a . char; for example,, x).
  • Type annotations. A type annotation in the form var: Type can be present in the lvalues or in the function argument definitions. The parser for type annotations is user-defined.
  • Boolean operations: ==, !=, &&, ||, !.
  • Order comparisons, that is, >, <, >=, and <= boolean ops.

Differences with Rust

(within shared syntax constructs; of course, Rust is much more expressive)

  • No keyword for assigning a variable (i.e., no let / let mut). There are no keywords in general.
  • Functions are only defined via the closure syntax.
  • There is “rest” destructuting for tuples and function arguments.
  • Type annotations are placed within tuple elements, for example, (x: Num, _) = y.
  • Object expressions are enclosed in #{ ... }, similarly to Rhai.

Crate features

  • std. Enables support of types from std, such as the Error trait, and propagates to dependencies.
  • num-complex. Implements NumLiteral for floating-point complex numbers (Complex32 and Complex64).
  • num-bigint. Implements NumLiteral for BigInt and BigUint from the num-bigint crate.


Using a grammar for arithmetic on real values.

use arithmetic_parser::{
    grammars::{F32Grammar, Parse, Untyped},
    NomResult, Statement, Expr, FnDefinition, LvalueLen,

const PROGRAM: &str = r#"
    // This is a comment.
    x = 1 + 2.5 * 3 + sin(a^3 / b^2 /* another comment */);
    // Function declarations have syntax similar to Rust closures.
    some_function = |a, b| (a + b, a - b);
    other_function = |x| {
        r = min(rand(), 0.5);
        r * x
    // Tuples and blocks are supported and have a similar syntax to Rust.
    (y, z) = some_function({ x = x - 0.5; x }, x);
    other_function(y - z)

let block = Untyped::<F32Grammar>::parse_statements(PROGRAM)?;
// First statement is an assignment.
    Statement::Assignment { ref lhs, .. } if *lhs.fragment() == "x"
// The RHS of the second statement is a function.
let some_function = match &block.statements[1].extra {
    Statement::Assignment { rhs, .. } => &rhs.extra,
    _ => panic!("Unexpected parsing result"),
// This function has a single argument and a single statement in the body.
    Expr::FnDefinition(FnDefinition { ref args, ref body, .. })
        if args.extra.len() == LvalueLen::Exact(2)
            && body.statements.is_empty()
            && body.return_value.is_some()



Grammar functionality and a collection of standard grammars.



Block of statements.


Tuple destructuring, such as (a, b, ..., c).


Function definition, e.g., |x, y| x + y.


Code span together with information related to where it is located in the code.


Object destructuring, such as { x, y: new_y }.


Single field in ObjectDestructure, such as x and y: new_y in { x, y: new_y }.


Object expression, such as #{ x, y: x + 2 }.


Parsing error with a generic code span.



Binary arithmetic operation.


Container for a code fragment that can be in one of the two states: either the code string is retained, or it is stripped away.


Parsing context.


Rest syntax, such as in (a,, b).


Parsing error kind.


Arithmetic expression with an abstract types for type annotations and literals.


Type of an Expr.


Assignable value.


Length of an assigned lvalue.


Type of an Lvalue.


Generic operation, either unary or binary.


Priority of an operation.


Statement: an expression or a variable assignment.


Type of a Statement.


Unary operation.


Description of a construct not supported by a certain module (e.g., interpreter or type inference).



Encapsulates stripping references to code fragments. The result can outlive the code.


Helper trait for Results with the error component that implements StripCode.



Checks if the provided string is a valid variable name.


Wrapper around parsers allowing to capture both their output and the relevant span.

Type Definitions


Error with code span available as a string reference.


Code span.


Value with an optional associated code span.


Parsing outcome generalized by the type returned on success.


Value with an associated code span.


Expr with the associated type and code span.


Lvalue with the associated code span.


Statement with the associated code span.