expr -- Evaluate algebraic expressions

Syntax: expr expr

Evaluates the expression, returning the result.

expr implements a little language that has a syntax separate from that of Molt. An expression is composed of values and operators, with parentheses for grouping, just as in C, Java, and so forth. Values consist of numeric and boolean literals, function calls, variable and command interpolations, and double-quoted and braced strings. Every value that looks like a number is treated as a number, and every value that looks like a boolean is treated as a boolean.

The operators permitted in expressions include most of those permitted in C expressions, with a few additional ones The operators have the same meaning and precedence as in C. Expressions can yield numeric or non-numeric results.

Integer computations are done with Rust's i64 type; floating-point computations are done with Rust's f64 type.


expr {1 + 1}

set x 7.5
set y 3.4
expr {$x + $y}

expr {[mycommand] + 2}

expr {2*(1 + abs($x))}

Operators and Precedence

The following table shows the operators in order of precedence.

Operators Details
- + ~ ! Unary plus, minus, bit-wise not, and logical not
* / % Multiplication, division, integer remainder
+ - Addition, subtraction
<< >> Left and right shift.
< > <= >= Ordering relations (see below)
== != Equality, inequality (see below)
eq ne String equality, inequality
in ni List inclusion, exclusion
& Bit-wise AND
^ Bit-wise exclusive OR
| Bit-wise OR
&& Logical AND, short circuiting
|| Logical OR, short circuiting
x ? y : z Ternary "if-then-else" operator.

Boolean Values

  • True values: any non-zero number, true, yes, on.
  • False values: zero, false, no, off.
  • Logical operators always return 0 or 1.
  • By convention, predicate commands also return 0 or 1.

Math Functions

Functions are written as "name(argument,...)". Each argument is itself a complete expression.

The following functions are available in Molt expressions:

abs(x) — Absolute value of x.

double(x) — Returns integer x as a floating-point value.

int(x) — Truncates floating-point value x and returns it as an integer.

round(x) — Rounds floating-point value x to the nearest integer and returns it as an integer.

TCL Liens

Expr Command Syntax: In standard TCL expr takes any number of arguments, which it concatenates into a single expression for evaluation. This means that variable and command interpolation is done twice, once by the TCL parser and once by expr, which hurts performance and can also be a source of subtle and confusing errors. Consequently it is almost always best to provide the expression as a single braced string, and so Molt's expr takes a single argument. This is unlikely to change.

Expression Syntax: Molt's expression parsing is meant to be consistent with TCL 7.6, with the addition of the TCL 8.x eq, ne, in, and ni operators.

  • Molt does not yet support the full range of math functions supported by TCL 7.6.
  • Molt does not yet do precise float-to-string-to-float conversions, per TCL 8.6. See
    "String Representation of Floating Point Numbers" on the Tcler's Wiki expr page.
  • Molt's handling of floating point arithmetic errors is still naive.

Integer Division: Integer division in Molt rounds down towards zero, following the example of Rust, Python, C99, and many other languages. Standard TCL rounds toward negative infinity, a decision that dates to a time when the C standard did not define the correct behavior and C compilers varied. It seems reasonable that an extension language should do something as basic as this in the same way as the host language.

Possible Futures: The following TCL 8.6 features are not on the road map at present, but might be added in the future.

  • Bignums
  • The exponential operator, **
  • The tcl::mathfunc:: namespace, and the ability to define new functions in TCL code.