Add use keyword

[alichraghi]

hello @edubart
i think the use would be useful
example:

local Config = @record {
  vsync: boolean,
  resizable: boolean,
  ...
}
local function run(config: Config)
  use config -- <----------------
  if vsync then
    print('vsync is on')
  elseif resizable then
    print('resizable is on')
  end
end

[ryanford]

The Lua way to do this would be to set a local _ENV table. Any plans to implement or is it implemented and not documented?

[edubart]

The language does not have this in its core and there is little motivation for that right now.

However is in the language goals to give means to implement such feature with some meta programming, this doable right now using the preprocessor, here is an example of that feature implemented:

-- Code to implement `using` macro.
##[[
local visitors = require 'nelua.analyzer'.visitors
local orig_visitors_Id = visitors.Id

-- Hook Id AST node visitor.
function visitors.Id(context, node)
  local name = node[1]
  if scope.usingfields then -- the scope has 'using' fields
    local usingsymbol = scope.usingfields[name] -- get symbol for the using field
    if usingsymbol and -- found a using field
       not context.scope.symbols[name] then -- no local with the same name
      -- transform this 'Id' node to 'DotIndex' node
      local newnode = aster.DotIndex{name, aster.Id{usingsymbol.name}}
      return context:transform_and_traverse_node(node, newnode)
    end
  end
  -- run the original Id visitor
  return orig_visitors_Id(context, node)
end

-- Using macro
function using(symbol)
  local scope = context.scope -- get current scope
  scope.usingfields = scope.usingfields or {} -- create using fields table
  for _,field in ipairs(symbol.type.fields) do
    scope.usingfields[field.name] = symbol -- associate a field with a symbol
  end
end
]]

local Config = @record {
  vsync: boolean,
  resizable: boolean,
}

local function run(conf: Config)
  ## using(conf)
  if vsync then
    print('vsync is on')
  elseif resizable then
    print('resizable is on')
  end
end

local conf: Config = {vsync = true}
run(conf)

There you have, the using macro doing what you wanted. You could move that preprocessor stuff to another file, like one called using.nelua, then you can just do:

require 'using'

local Config = @record {
  vsync: boolean,
  resizable: boolean,
}

local function run(conf: Config)
  ## using(conf)
  if vsync then
    print('vsync is on')
  elseif resizable then
    print('resizable is on')
  end
end

local conf: Config = {vsync = true}
run(conf)