atomic_types.md

Atomic types

Atomic types are the basic building block of all type information used in Psalm. Multiple atomic types can be combined, either with union types or intersection types. Psalm allows many different sorts of atomic types to be expressed in docblock syntax:

Scalar types

• int
• positive-int
• float
• string
• class-string and class-string<Foo>
• trait-string
• enum-string
• callable-string
• numeric-string
• literal-string
• bool
• array-key
• numeric
• scalar

Object types

• object
• Exception, Foo\MyClass and Foo\MyClass
• Generator

Array types

• array & non-empty-array
• array<int, string>
• string[]
• list & non-empty-list
• list<string>
• array{foo: int, bar: string}
• callable-array

Callable types

• callable, Closure and callable(Foo, Bar):Baz

Value types

• null
• true, false
• 6, 7.0, "forty-two" and 'forty two'
• Foo\Bar::MY_SCALAR_CONST

Magical types

• (T is true ? string : bool)
• key-of<Foo\Bar::ARRAY_CONST>
• value-of<Foo\Bar::ARRAY_CONST>
• T[K]

Top types, bottom types

mixed

This is the top type in PHP's type system, and represents a lack of type information. Psalm warns about mixed types when the reportMixedIssues flag is turned on, or when you're on level 1.

never

It can be aliased to no-return or never-return in docblocks. Note: it replaced the old empty type that used to exist in Psalm

This is the bottom type in PHP's type system. It's used to describe a type that has no possible value. It can happen in multiple cases:

• the actual never type from PHP 8.1 (can be used in docblocks for older versions). This type can be used as a return type for functions that will never return, either because they always throw exceptions or always exit()
• an union type that have been stripped for all its possible types. (For example, if a variable is string|int and we perform a is_bool() check in a condition, the type of the variable in the condition will be never as the condition will never be entered)
• it can represent a placeholder for types yet to come — a good example is the type of the empty array [], which Psalm types as array<never, never>, the content of the array is void so it can accept any content
• it can also happen in the same context as the line above for templates that have yet to be defined

Other

• iterable - represents the iterable pseudo-type. Like arrays, iterables can have type parameters e.g. iterable<string, Foo>.
• void - can be used in a return type when a function does not return a value.
• resource represents a PHP resource.