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Discourage definition of custom primitive types #35526

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Apr 20, 2020
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Discourage definition of custom primitive types #35526

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Discourage definition of custom primitive types
cmcaine Apr 20, 2020
df94011
Caution that LLVM only likes some type sizes
cmcaine Apr 20, 2020
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13 changes: 11 additions & 2 deletions doc/src/manual/types.md
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Expand Up @@ -236,6 +236,14 @@ of function arguments that are containers of abstract types; see [Performance Ti

## Primitive Types

!!! warning
It is almost always preferable to wrap an existing primitive type in a new
composite type than to define your own primitive type.

This functionality exists to allow Julia to bootstrap the standard primitive
types that LLVM supports. Once they are defined, there is very little reason
to define more.

A primitive type is a concrete type whose data consists of plain old bits. Classic examples of primitive
types are integers and floating-point values. Unlike most languages, Julia lets you declare your
own primitive types, rather than providing only a fixed set of built-in ones. In fact, the standard
Expand Down Expand Up @@ -273,8 +281,9 @@ a name. A primitive type can optionally be declared to be a subtype of some supe
is omitted, then the type defaults to having `Any` as its immediate supertype. The declaration
of [`Bool`](@ref) above therefore means that a boolean value takes eight bits to store, and has
[`Integer`](@ref) as its immediate supertype. Currently, only sizes that are multiples of
8 bits are supported. Therefore, boolean values, although they really need just a single bit,
cannot be declared to be any smaller than eight bits.
8 bits are supported and you are likely to experience LLVM bugs with sizes other than those used above.
Therefore, boolean values, although they really need just a single bit, cannot be declared to be any
smaller than eight bits.

The types [`Bool`](@ref), [`Int8`](@ref) and [`UInt8`](@ref) all have identical representations:
they are eight-bit chunks of memory. Since Julia's type system is nominative, however, they
Expand Down