// implement a type that can be unvalid or valid. 
// It's a replacement of Optional, but can be allocated on stack, so no heap allocaton needed.
// When unvalid, it should be not possible to get unvalid value, thats why abstract type is used.
// bool b: true means valid, false unvalid
abst@ype valid(a:t@ype,b:bool) = (a,bool b)

typedef Valid(a:t@ype) = [b : bool] valid(a,b) 

// Wraps valid value.
fn {a:t@ype} create_valid(value : a) : valid(a,true)

// Creates unvalid type.
fn {a:t@ype} create_unvalid() : valid(a,false)

// Only function that allows getting value. Only accepts valid values. 
fn {a:t@ype} unwrap_valid(valid(a,true)) : a