local T = {} -- types
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-- istype[o] iff o represents a type (i.e. set of values)
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T.istype = {}
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-- iserror[o] iff o represents an error type (created via T.error).
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T.iserror = {}
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-- istabletype[o] iff o represents a table type (created by T.table).
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T.istabletype = {}
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-- Number type
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T.number = {}
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setmetatable(T.number, T.number)
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function T.number.__tostring(self)
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return 'number'
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end
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T.istype[T.number] = true
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-- String type
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T.string = {}
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setmetatable(T.string, T.string)
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function T.string.__tostring(self)
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return 'string'
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end
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T.istype[T.string] = true
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-- Boolean type
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T.boolean = {}
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setmetatable(T.boolean, T.boolean)
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function T.boolean.__tostring(self)
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return 'boolean'
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end
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T.istype[T.boolean] = true
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-- Table type
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function T.table(t)
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T.istype[t] = true
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T.istabletype[t] = true
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return t
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end
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-- Universal type. This is a superset of all other types.
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T.universal = {}
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setmetatable(T.universal, T.universal)
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function T.universal.__tostring(self)
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return 'unknown'
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end
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T.istype[T.universal] = true
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-- nil type. Represents `nil` but can be stored in tables.
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T['nil'] = {}
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setmetatable(T['nil'], T['nil'])
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T['nil'].__tostring = function(self)
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return 'nil'
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end
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T.istype[T['nil']] = true
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-- None type. Represents a non-existent value, in a similar way
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-- that `none` is used differently from `nil` in the Lua C API.
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T.none = {}
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setmetatable(T.none, T.none)
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function T.none.__tostring(self)
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return 'none'
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end
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T.istype[T.none] = true
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-- Error type
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local CError = {}; CError.__index = CError
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function CError.__tostring(self) return "error:" .. tostring(self.value) end
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function T.error(val)
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local self = setmetatable({value=val}, CError)
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T.istype[self] = true
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T.iserror[self] = true
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return self
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end
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-- Gets a type that is a superset of the two given types.
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function T.superset_types(a, b)
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if T.iserror[a] then return a end
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if T.iserror[b] then return b end
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if rawequal(a, b) then -- note: including nil == nil
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return a
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elseif type(a) == 'string' or a == T.string then
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if type(b) == 'string' or b == T.string then
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return T.string
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else
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return T.universal
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end
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elseif type(a) == 'number' or a == T.number then
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if type(b) == 'number' or b == T.number then
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return T.number
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else
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return T.universal
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end
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elseif type(a) == 'boolean' or a == T.boolean then
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if type(b) == 'boolean' or b == T.boolean then
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return T.boolean
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else
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return T.universal
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end
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else
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return T.universal -- IMPROVE
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end
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end
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--[[TESTS:
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assert(T.superset_types(2, 2) == 2)
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assert(T.superset_types(2, 3) == T.number)
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assert(T.superset_types(2, T.number) == T.number)
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assert(T.superset_types(T.number, T.string) == T.universal)
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print 'DONE'
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--]]
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-- Determines whether type `o` certainly evaluates to true (true),
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-- certainly evaluates to false (false) or could evaluate to either
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-- true of false ('?').
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function T.boolean_cast(o)
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if T.iserror[o] then -- special case
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return '?'
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elseif o == nil or o == false or o == T['nil'] then -- all subsets of {nil, false}
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return false
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elseif o == T.universal or o == T.boolean then -- all supersets of boolean
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return '?'
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else -- all subsets of universal - {nil, false}
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return true
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end
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end
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return T
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