Question about polymorphic function and undeclared symbol

[codehz]

I created a function that will print enum's name

local enumType = #[concept(function(attr)
  return attr.type.is_enum
end)]#

-- local Boom = @enum{ A = 0 }

global function printEnum(value: enumType): string
  ##[[
    local type = value.type.symbol
    local node = aster.Switch{aster.Id{"value"}}
    local arr = {}
    for _, field in ipairs(type.value.fields) do
      table.insert(arr, {aster.DotIndex{field.name, aster.Id{type.name}}})
      table.insert(arr, aster.Block{aster.Return{aster.String{field.name}}})
    end
    table.insert(node, arr)
    table.insert(node, aster.Block{aster.Return{aster.String{"Unknown"}}})
    inject_astnode(node)
  ]]
end

-- it doesn't work here
local Boom = @enum{ A = 0 }
print(printEnum(Boom.A))

but when I use it, the compiler say undeclared symbol 'Boom'

[codehz]

Ok, that's the workaround, is it correct?

local enumType = #[concept(function(attr)
  return attr.type.is_enum
end)]#

global function printEnum(value: enumType): string
  ## local type = value.type.symbol
  local TargetType = #[value.type]#
  ##[[
    local node = aster.Switch{aster.Id{"value"}}
    local arr = {}
    for _, field in ipairs(type.value.fields) do
      table.insert(arr, {aster.DotIndex{field.name, aster.Id{"TargetType"}}})
      table.insert(arr, aster.Block{aster.Return{aster.String{field.name}}})
    end
    table.insert(node, arr)
    table.insert(node, aster.Block{aster.Return{aster.String{"Unknown"}}})
    inject_astnode(node)
  ]]
end

[edubart]

The problem with your first example is that the symbol is not visible due to declaration order, your workaround is correct, saving the value.type to the local TargetType makes the symbol always visible inside printEnum.

Congrats on being able to make some interesting function with meta programming!

Another possible implementation of this is the following:

local enumType = #[concept(function(attr)
  return attr.type.is_enum
end)]#

global function printEnum(value: enumType): string
  ## for i,field in ipairs(value.type.fields) do
  if value == #[field.value]# then
    return #[field.name]#
  end
  ## end
  return 'Unknown'
end

-- it doesn't work here
local Boom = @enum{ A = 0 }
print(printEnum(Boom.A))

This doesn't use the switch statement, but in most cases the C compiler will be able to optimize a series of ifs into a switch.