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Clarify type parameters #35971

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c866dea
Clarify type parameters
ettersi May 21, 2020
b62cbeb
Fix line breaks
ettersi Dec 16, 2020
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20 changes: 12 additions & 8 deletions doc/src/manual/types.md
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Expand Up @@ -41,11 +41,9 @@ up front are:
type when the program is running. This is called a "run-time type" in object-oriented languages
where the combination of static compilation with polymorphism makes this distinction significant.
* Only values, not variables, have types -- variables are simply names bound to values.
* Both abstract and concrete types can be parameterized by other types. They can also be parameterized
by symbols, by values of any type for which [`isbits`](@ref) returns true (essentially, things
like numbers and bools that are stored like C types or `struct`s with no pointers to other objects),
and also by tuples thereof. Type parameters may be omitted when they do not need to be referenced
or restricted.
* Both abstract and concrete types can be parameterized by types, symbols, numbers and a
[few other objects](@ref man-parametric-types). Type parameters may be omitted when they do not need to be referenced or
restricted.

Julia's type system is designed to be powerful and expressive, yet clear, intuitive and unobtrusive.
Many Julia programmers may never feel the need to write code that explicitly uses types. Some
Expand Down Expand Up @@ -529,7 +527,7 @@ types in other languages. Declaring a function argument or a field as `Union{T,
setting it either to a value of type `T`, or to `nothing` to indicate that there is no value.
See [this FAQ entry](@ref faq-nothing) for more information.

## Parametric Types
## [Parametric Types](@id man-parametric-types)

An important and powerful feature of Julia's type system is that it is parametric: types can take
parameters, so that type declarations actually introduce a whole family of new types -- one for
Expand All @@ -546,8 +544,14 @@ because Julia is a dynamically typed language and doesn't need to make all type
time, many traditional difficulties encountered in static parametric type systems can be relatively
easily handled.

All declared types (the `DataType` variety) can be parameterized, with the same syntax in each
case. We will discuss them in the following order: first, parametric composite types, then parametric
All declared types (the `DataType` variety) can be parameterized by the following objects.
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All declared types (the `DataType` variety) can be parameterized by the following objects.
All declared types (the `DataType` variety) can be parameterized by the following objects (as well as a few other, more niche objects).


* A type, e.g. `Foo{Int}`.
* Symbols, e.g. `Foo{:bar}`.
* Objects of `isbits` type, e.g. `Foo{42}` or `Foo{3.14}`.
* Tuples of the above, e.g. `Foo{(Int,:bar,42)}`.

We will discuss parametric types in the following order: first, parametric composite types, then parametric
abstract types, and finally parametric primitive types.

### Parametric Composite Types
Expand Down