doc/glossary.md

# 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](/doc/note/coroutines.md) 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](/doc/note/slices-arrays-and-tables.md)-typed variable `s`. Dependent
types are *a* way to implement [bounds checking](/doc/note/bounds-checking.md),
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](/doc/note/coroutines.md)
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](/doc/note/facts.md) note for more details.

#### Flick

A unit of time. One [flick](https://github.com/OculusVR/Flicks) (frame-tick) is
`1 / 705_600_000` of a second.

#### Interval Arithmetic

Arithmetic on numerical ranges. See the [interval
arithmetic](/doc/note/interval-arithmetic.md) 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](https://github.com/jbangert/nail/issues/7).

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](/doc/note/ranges-and-rects.md). 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](/doc/note/facts.md) at a given point in a program.

#### Utility Type

An empty struct (with no fields) used as a placeholder. Every written-in-Wuffs
function is a method: the receiver is mandatory. A Wuffs utility type's methods
are to its package what C++ or Java's static methods are to its class.

"Utility type" is merely a Wuffs naming convention. For example, Wuffs' `base`
package has a type called `base.utility`, similar to how the `zlib` package has
a type called `zlib.decoder`. Unlike "dependent type" or "refinement type",
"utility type" is not a phrase used in programming language type theory.