expr - Expression Evaluation

Evaluates a mathematical or logical expression.

Syntax

tcl

expr arg ?arg ...?

Parameters

arg: One or more arguments forming the expression

Description

The expr command evaluates its arguments as a mathematical expression and returns the result. Arguments are concatenated with spaces before evaluation.

Summary of Our Implementation

Feather implements a comprehensive expression evaluator. The implementation uses a recursive descent parser with proper operator precedence and supports:

Full arithmetic operations with integer and floating-point numbers
Boolean and comparison operators
Bitwise operations
Variable substitution ($var, ${var})
Command substitution ([cmd args])
Ternary conditional operator (?:)
Math function calls (via tcl::mathfunc:: namespace)
Short-circuit evaluation for &&, ||, and ?:
Comments starting with #

Supported Features

Operands

Numeric literals: integers (decimal, hex 0x, binary 0b, octal 0o), floating-point (3.14, .5, 1e10, 3.14e-5)
Digit separators: underscores in numeric literals (100_000_000, 0xffff_ffff)
Boolean literals: true, false, yes, no, on, off (case-insensitive)
Special float values: Inf, -Inf, NaN recognized
Decimal prefix: 0d prefix for explicit decimal (e.g., 0d123)
Variables: $name, ${name}, including namespace-qualified names
Command substitution: [command args]
Braced strings: {literal string}
Quoted strings: "string with $var and [cmd] substitution"
Parenthesized expressions: (expr)
Comments: # to end of line

Operators (in precedence order, lowest to highest)

Operator(s)	Description
`?:`	Ternary conditional (right-to-left)
`\|\|`	Logical OR (short-circuit)
`&&`	Logical AND (short-circuit)
`\|`	Bitwise OR
`^`	Bitwise XOR
`&`	Bitwise AND
`==` `!=`	Numeric equality (with string fallback)
`eq` `ne`	String equality
`<` `>` `<=` `>=`	Numeric comparison (with string fallback)
`lt` `gt` `le` `ge`	String comparison
`in` `ni`	List containment
`<<` `>>`	Bitwise shift
`+` `-`	Addition, subtraction
`*` `/` `%`	Multiplication, division, modulo
`**`	Exponentiation (right-to-left)
`-` `+` `~` `!`	Unary minus, plus, bitwise NOT, logical NOT

Math Functions (via `tcl::mathfunc::`)

Function	Description
`abs(x)`	Absolute value
`acos(x)`	Arc cosine
`asin(x)`	Arc sine
`atan(x)`	Arc tangent
`atan2(y, x)`	Arc tangent of y/x
`bool(arg)`	Convert to boolean (0 or 1)
`ceil(x)`	Ceiling
`cos(x)`	Cosine
`cosh(x)`	Hyperbolic cosine
`double(x)`	Convert to double
`entier(x)`	Convert to integer (same as `int()` in Feather)
`exp(x)`	Exponential
`floor(x)`	Floor
`fmod(x, y)`	Floating-point modulo
`hypot(x, y)`	Hypotenuse
`int(x)`	Convert to integer
`isfinite(x)`	Test if finite
`isinf(x)`	Test if infinite
`isnan(x)`	Test if NaN
`isnormal(x)`	Test if normal
`issubnormal(x)`	Test if subnormal
`isunordered(x, y)`	Test if either is NaN
`log(x)`	Natural logarithm
`log10(x)`	Base-10 logarithm
`max(arg, ...)`	Maximum of arguments
`min(arg, ...)`	Minimum of arguments
`pow(x, y)`	Power
`round(x)`	Round to nearest integer
`sin(x)`	Sine
`sinh(x)`	Hyperbolic sine
`sqrt(x)`	Square root
`tan(x)`	Tangent
`tanh(x)`	Hyperbolic tangent
`wide(x)`	Convert to wide integer

Unsupported Features

These functions are NOT implemented in Feather:

Function	Reason
`isqrt(x)`	Requires arbitrary precision integers (bignums), which Feather does not support
`rand()`	Random number generation is outside Feather's scope as an embeddable interpreter
`srand(seed)`	Random number generation is outside Feather's scope as an embeddable interpreter

Implementation Notes

Integer Division and Modulo

TCL specifies that integer division truncates toward negative infinity (floor division), and the modulo result has the same sign as the divisor. Feather uses C's native integer division which truncates toward zero. This may produce different results for negative operands.

Exponentiation Limits

TCL documents a maximum exponent value of 268435455 when the base is an integer greater than 1. Feather does not enforce this limit explicitly and may overflow silently or produce incorrect results for very large exponents.

Arbitrary Precision

TCL's entier and isqrt functions support arbitrary precision integers. Feather uses fixed 64-bit integers throughout, so very large integer results may overflow.

Type Preservation

TCL is careful to preserve integer types when possible (e.g., 5 / 4 returns integer 1). Feather generally follows this behavior but may differ in edge cases.

String to Number Conversion

Feather Script

Canvas

expr - Expression Evaluation ​

Syntax ​

Parameters ​

Description ​

Summary of Our Implementation ​

Supported Features ​

Operands ​

Operators (in precedence order, lowest to highest) ​

Math Functions (via tcl::mathfunc::) ​

Unsupported Features ​

Implementation Notes ​

Integer Division and Modulo ​

Exponentiation Limits ​

Arbitrary Precision ​

Type Preservation ​

String to Number Conversion ​

Examples ​

Interactive Playground

Interactive Playground

Interactive Playground

Interactive Playground

Interactive Playground

Interactive Playground

Interactive Playground

See Also ​

expr - Expression Evaluation

Syntax

Parameters

Description

Summary of Our Implementation

Supported Features

Operands

Operators (in precedence order, lowest to highest)

Math Functions (via `tcl::mathfunc::`)

Unsupported Features

Implementation Notes

Integer Division and Modulo

Exponentiation Limits

Arbitrary Precision

Type Preservation

String to Number Conversion

Examples

See Also