xtype (halted)

= xtype
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*xtype*, or Extended Type, is a dynamic type system library for Lua.

Sometimes, advanced type checking code is needed to implement specific abstractions; *xtype* aims to fulfill that need.

.Incentives:
Simplification:: Prevent writing redundant type checking code.
Interoperability:: Having a general low-level type system library can ease the interactions of third-party code.
Metaprogramming / Genericity:: Exploit the dynamic nature of Lua to generate definitions on-the-fly.

See link:src[].

== Concepts

A non-primitive type can inherit from other types, building a stack of types. This stack defines the order in which the types are evaluated, from the terminal type to the least specific inherited types.

=== Multifunction

A multifunction is a function that can have multiple definitions, each associated to a specific signature. When called, it resolves the call by selecting a matching definition from the call signature.

WARNING: Ambiguities are ignored. If two definitions are candidates with the same weight, one will be selected arbitrarily.

==== Metaprogramming

We can use Lua as its own metaprogramming language: we can generate Lua code from Lua.

A multifunction can have generators that are called when no definitions could be found, allowing to generate definitions for specific call signatures on-the-fly.

    __index = multifunction,
    __call = default_call
  }
  return setmetatable({
    hsign = new_hsign(),
    definitions = {}, -- map of sign hash => {.f, .sign}
    generators = {}, -- set of generator functions
    candidates = {}, -- cached candidates, map of call sign hash => {def, dist}
    max_calln = 0, -- maximum call parameters
    call = default_call
  }, multifunction_mt)
end

-- Code generation tools.