Language extensions

PxIgnis runs on PxLuaNova, a Lua 5.2 fork. This page documents extensions that aren’t part of standard Lua 5.2.

Lambda literal \{ ... }

PxLuaNova adds a non-standard syntax for inline anonymous functions. Opt in per file by placing the magic comment on line 1:

--# nova syntax

This enables the \ + { two-character sequence to be recognized as a lambda literal. Without the pragma, \ outside a string literal is a syntax error.

Named-arg lambda

\{ args -> body } — arguments followed by -> and the body.

I tried to implement implicit return like \{ 5 } that returns 5, but in Lua is too buggy. So just use return.

Lambda

--# nova syntax
local double = \{ x -> return x * 2 }
print(double(21))  -- 42

Function

local function double(x)
  return x * 2
end
print(double(21))

Multi-arg:

Lambda

--# nova syntax
local sum = \{ a, b, c -> return a + b + c }
print(sum(1, 2, 3))  -- 6

Function

local function sum(a, b, c)
  return a + b + c
end
print(sum(1, 2, 3))

Zero-arg

No parameters. Body is always a chunk; use return for values:

Lambda

--# nova syntax
local answer = \{ return 40 + 2 }
print(answer())  -- 42

Function

local function answer()
  return 40 + 2
end
print(answer())

Body can contain statements:

Lambda

--# nova syntax
local get = \{
  print("side effects OK")
  return 42
}
print(get())  -- 42

Function

local function get()
  print("side effects OK")
  return 42
end
print(get())

Trailing-block sugar

\{ ... } after a function call appends the lambda as the last argument. The body is always parsed as a chunk (no implicit return). This is especially useful with register():

Lambda

--# nova syntax
register("greet") \{ ctx ->
  ctx.player:send("hello!")
}

Function

register("greet", function(ctx)
  ctx.player:send("hello!")
end)

Works with any function, not just register():

--# nova syntax
local results = {}
local function each(t, fn)
  for i, v in ipairs(t) do
    fn(v, i)
  end
end
each({ "a", "b", "c" }) \{ v, i ->
  results[i] = v .. i
}
-- results == { "a1", "b2", "c3" }

Closures

Lambdas capture local variables from the enclosing scope, just like function:

--# nova syntax
function makeCounter()
  local n = 0
  return \{
    n = n + 1
    return n
  }
end

local c = makeCounter()
print(c())  -- 1
print(c())  -- 2

Standard library additions

The Lua standard library in PxLuaNova includes some Lua 5.3+ functions not present in standard Lua 5.2:

Function	Description
math.type(x)	Returns "integer" or "float"
math.atan(y, x)	Two-argument arctangent (Lua 5.3+)
math.min(...)	Variadic minimum
math.max(...)	Variadic maximum