nelua trait example

nelua_trait_example.lua

-- based from https://github.com/edubart/nelua-lang/blob/master/lib/allocators/interface.nelua

-- a trait impl. can be just a macro which should receive a record as parameter
## local function sum_fields_trait(R)
  -- ensure that the argument is really a record
  ##[[ staticassert(
    R and R.value and R.value.is_record,
    "parameter passed to 'sum_fields' is not a record"
  )]]

  -- mark on the received type that implements the the "sum_fields" trait
  ## R.value.is_sum_fields = true

  -- type alias of the received type
  local R: type = #[R]#
  
  -- only implements sum_fields when there aren't already a "sum_fields" metafield
  -- (a function declared on a record is a metafield of this record)
  ## if not R.value.metafields.sum_fields then
  -- default "sum_fields" implementation
  function R:sum_fields(): number
    local result: number = 0

    -- generates the sum code, adding one sum
    -- for each field with is_arithmetic trait
    ## for _, field in ipairs(R.value.fields) do
      ## if field.type.is_arithmetic then
        result = result + self.#|field.name|#
      ## end
    ## end

    return result
  end
  ## end
## end

----------
-- TEST --
----------

local A = @record{ x: integer, s: stringview }
local B = @record{ a: integer, b: integer}

-- this will "override" the default 
function B:sum_fields(): number
  return (self.a + self.b) * -1000
end

## sum_fields_trait(A)
## sum_fields_trait(B)

local a: A = {10, "hello"}
local b: B = {10, 20}

print('a sum: ', a:sum_fields())
print('b sum: ', b:sum_fields())

-- Prints:
-- a sum: 	10
-- b sum: 	-30000