Declaration and assignment

A node is two slots, a type slot and a value slot, and Logos construction is immutable by default. Three operators write those slots.

Operator Meaning Example
: Declare: introduce a name and set its type slot, leaving the value undefined a : i32
:= Declare with a value: introduce a name and set its value slot, inferring the type a := 32
= Reassign the value of an already-declared name a = a + 1

: writes the type slot; := and = write the value slot. A bare a : declares a name whose two slots are both undefined. There is no type = value form.

mut is a type modifier

mut is not a qualifier on a variable; it is a modifier on a type (mut T is "the type of a mutable T"), the same mut as in &mut T. The two slots are independent permissions, so a value construction spells out four states:

i32 32              # frozen type, frozen value      : a plain constant
mut i32 32          # frozen type, mutable value     : an ordinary variable
mut type i32 32     # mutable type, frozen value     : reinterpret-only
mut type mut i32 32 # mutable type, mutable value    : full structural mutability

So an ordinary mutable variable is declared a := mut i32 32, and a = a + 1 reassigns it.