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kc3-lang/kc3/doc/1_KC3/1.06_Variable.en.md

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  • # 1.6 Variables

Variables in KC3 can be defined using the litteral value for a variable
which is always `?`. You can cast this litteral value and it will not
really be casted but it will give you a typed variable litteral value.
E.g. `(List) ?`.
The typed variable litteral value will only accept to be set once to
one value of the variable's type (in this example the type of a linked
list).

It's actually a syntax so you cannot rename `?` by mistake and
so is an easy task to do static analysis of variable creation.

The default type for a variable which you can also specify explicitly
is `Tag` which is an enum-tagged union type of any other KC3 types
currently defined in the environment. So `?` is exactly equivalent to
`(Tag) ?` and they will both accept to be set once to one value of any
type.

A variable is settable once and cannot be changed afterwards (there is
an exception if you write C code and link to it but it is not easy nor
silent).

This way you do not need to lock or trust any behaviour, once your
variable is set to a value the value of the variable will never change,
it really is read-only.

You can also use the assignment operator which is `<-` which in turn calls
`tag_init_copy`. It works like the C assignment operator (`=`).

Examples :
```elixir
# Declare a unsigned byte 8 bits variable "x".
x = (U8) ?
# Set the variable "x" to zero.
x <- 0
# Allocate again for the same binding name "x"
x = (U8) ?
# Also set the new variable "x" to zero with just one Unicode symbol
# that is AltGr+Y on my keyboard.
x ← 0
```


## So how do I change anything if it is read-only ?

You can always reset an existing binding at will to another variable
litteral and another variable will be created for the same name and it
will be in a different memory location, settable once and then
read-only again so you can use it without locking.

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