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Lua Academy · Lesson

Operator Overloading for DSL Fluency

Use metamethods to build fluent builder APIs and chainable expressions.

Metamethods Enable Operator DSLs

By overloading arithmetic and comparison metamethods, you can make Lua expressions build domain objects instead of computing values.

A Query Builder with Metamethods

Define a QueryExpr type where >, <, and == build condition objects.

local QueryExpr = {}
QueryExpr.__index = QueryExpr
function QueryExpr.new(field)
  return setmetatable({field=field, conditions={}}, QueryExpr)
end
QueryExpr.__lt = function(self, val)
  return {field=self.field, op="<", value=val}
end
QueryExpr.__gt = function(self, val)
  return {field=self.field, op=">", value=val}
end

All lessons in this course

  1. DSL Design Principles in Lua
  2. Operator Overloading for DSL Fluency
  3. Building a Config DSL
  4. Simple Expression Parser
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