Operators

A table-first reference. The operators manual page covers the same material with prose and worked examples.

Arithmetic

Operator	Name	Types	Example
+	Addition	int, float, string, list	1 + 2, "a" + "b"
-	Subtraction	int, float	5 - 3
*	Multiplication	int, float	3 * 4
/	Division	int, float	10 / 3
%	Remainder	int, float	7 % 3
**	Exponentiation	int, float	2 ** 10
- (unary)	Negate	int, float	-x

Integer / truncates toward zero. Convert with to_float() for a fractional result. + is overloaded for string and list concatenation.

Comparison

Operator	Meaning
==	Equal
!=	Not equal
<	Less than
<=	Less than or equal
>	Greater than
>=	Greater than or equal

Comparisons return bool. Equality is structural for primitives, structs, and collections, and reference identity for function values.

Logical

Operator	Meaning	Notes
&&	AND	Short-circuits
||	OR	Short-circuits
!	NOT	Unary prefix

Membership and type tests

Operator	Meaning
in	Element of list / key of map / member of set / substring of string
is	Type test (narrows in branches)

print(2 in [1, 2, 3])             // true
print("k" in {"k": "v"})          // true
print("x" is string)              // true

Range

Operator	Meaning
..	Half-open range, exclusive on the right
..=	Inclusive on both ends

for i in 0..3   { print(i) }      // 0, 1, 2
for i in 0..=3  { print(i) }      // 0, 1, 2, 3

Pipeline

Operator	Meaning
|	Insert the left-hand value as the first argument to the right-hand call

let total =
  [1, 2, 3]
  | map(fun(n: int): int => n * 2)
  | reduce(0, fun(a: int, b: int): int => a + b)

Optional access and defaults

Operator	Meaning
?.	Field / method access; returns nil if the receiver is nil
??	Default value if the left-hand side is nil

let name = user?.name ?? "anonymous"

Set operators

Operator	Meaning
union	All elements from both inputs
union all	Concatenation preserving duplicates
intersect	Elements in both inputs
except	Elements in left but not in right

let a = {1, 2, 3}
let b = {2, 3, 4}
print(a union b)       // {1, 2, 3, 4}
print(a intersect b)   // {2, 3}
print(a except b)      // {1}

Attribute and index

Operator	Meaning
.	Field or method access
[i]	Index a list or string, or look up a key in a map
[i..j]	Slice a list or string
[i..], [..j]	Slice with one end open

Assignment

Operator	Meaning
=	Assign
+=, -=, *=, /=, %=, **=	Compound assignments

Precedence

Highest (binds tightest) to lowest. Operators on the same row associate as shown.

Level	Operators	Associativity
1	?., […], (…), .	Left
2	unary -, !	Right
3	**	Right
4	*, /, %	Left
5	+, -	Left
6	.., ..=	None
7	<, <=, >, >=	None
8	==, !=, in, is	None
9	&&	Left
10	||	Left
11	??	Right
12	| (pipeline)	Left
13	union, intersect, except	Left
14	=, +=, -=, *=, /=, %=	Right

When in doubt, parenthesize. Mochi does not warn about redundant parentheses, and they cost nothing at runtime.