# Glossary

#### Coroutine

A function that can suspend execution and, when called again later, resume where it left off, in the middle of the function body. See the coroutines note for more details.

#### Dependent Type

A type that depends on another value. For example, a variable `n`‘s type might be “the length of `s`”, for some other slice-typed variable `s`. Dependent types are a way to implement bounds checking, but they’re not the only way. Wuffs does not use them.

#### Effect

An extension of the type system, applied to functions. Wuffs has three effect categories: pure, impure and coroutine. Pure means that the function has no side effects - it does not change any observable state. The other two categories may have side effects, with coroutines also being able to suspend and resume.

Impure functions are marked with a `!` at their definition and at call sites. Coroutines are similarly marked, with a `?`. Pure functions have no mark.

#### Fact

A boolean expression (e.g. `x > y`) that happens to be true at a given point in a program. See the facts note for more details.

#### Flick

A unit of time. One flick (frame-tick) is `1 / 705_600_000` of a second.

#### Interval Arithmetic

Arithmetic on numerical ranges. See the interval arithmetic note for more details.

#### Modular Arithmetic

Arithmetic that wraps around at a certain modulus, such as `256` for the `base.u8` type. For example, if `x` is a `base.u8` with value `200`, then `(x + 70)` has value `270` and would overflow, but `(x ~mod+ 70)` has value `14`.

#### Operator Precedence

A ranking of arithmetic operators so that an expression like `a + b * c` is unambiguous (for the computer), equivalent to exactly one of `(a + b) * c` or, more conventionally, `a + (b * c)`. While good for brevity, Wuffs does not have operator precedence: the bare `a + b * c` is an invalid Wuffs expression and the parentheses must be explicit. The ambiguity (for the human) can be a source of bugs in other security-concious file format parsers.

Some binary operators (`+`, `*`, `&`, `|`, `^`, `and`, `or`) are also associative: `(a + b) + c` and `a + (b + c)` are equivalent, and can be written in Wuffs as `a + b + c`.

#### Refinement Type

A basic type further constrained to a subset of its natural range. For example, if `x` has type `base.u8[0 ..= 99]` then it is an unsigned 8-bit integer whose value must be less than `100`. Without the refinement, `x` could be as high as `255`.

`base.u8[0 ..= 99]` and `base.u32[0 ..= 99]` are two different types. They can have different run-time representations.

Non-nullable pointer types can be thought of as a refinement of regular pointer types, where the refined range excludes the `nullptr` value.

#### Saturating Arithmetic

Arithmetic that stops at certain bounds, such as `0` and `255` for the `base.u8` type. For example, if `x` is a `base.u8` with value `200`, then `(x + 70)` has value `270` and would overflow, but `(x ~sat+ 70)` has value `255`.

#### Situation

The set of facts at a given point in a program.