Preprocessing

Pluto provides some powerful constructs which allow you to write code that will never be seen at runtime.

Function calls

Certain standard library functions can be called at compile-time, as long as the arguments are also known at compile-time, by using the "$" symbol before the function call.

pluto
local runtime = require("crypto").joaat("Hello, World!")
local compile = $crypto.joaat("Hello, World!")
assert(runtime == compile)

If you're unfamiliar with compile-time evaluation, this is essentially what the code turns into:

pluto
local runtime = require("crypto").joaat("Hello, World!")
local compile = 847757641 -- The JOAAT hash of "Hello, World!"
assert(runtime == compile)

This is available on preloaded Pluto libraries, such as:

• base32
• base58
• base64
• crypto
• json
• url

And on the following functions:

• io.contents
• tostring
• utostring
• tonumber
• utonumber
• type
• assert

Variables

Compile-time constant variables can be defined via the $define statement:

pluto
$define a = 123

This may seem identical to a local declared with the const attribute:

pluto
local a <const> = 123

However, $define also enforces that the assigned variable is a compile-time constant:

pluto
local a <const> = {}
$define a = {} -- Error: variable was not assigned a compile-time constant value

Conditionals

If there is certain code you only want to have compiled in for a certain build, such as a debug build, you can use compile-time conditionals:

pluto
$define DEBUG = true
$if DEBUG then
    print("Script running in debug mode")
$else
    print("Script running in release mode")
$end

In this case, only one of the two paths will be compiled in; the rest will not take up any space.

Aliases

Preprocessor aliases are similar to C/C++ macros. For example, you can define an alias for a keyword:

pluto
$alias let = local
let a = 1
print(a) --> 1

or write simple functions which will be fully inlined at the call site:

pluto
$alias add(a, b) = a + b
print(add(1, 2)) --> 3

If you want to write an alias over multiple lines, you can use a backslash to continue it:

pluto
$alias seq = "a"  \
             ..   \
             "b"  \
             ..   \
             "c"
print(seq) --> abc

$include

To reuse code for Pluto's parse-time features such as type hinting, you can include a common 'header' file.

header.pluto
pluto
$declare _VERSION: string
$declare _PVERSION: string

index.pluto
pluto
$include "header.pluto"
local _: number = _VERSION -- type mismatch

$haltcompiler

This statement will stop the parser processing further code.

pluto
print(io.contents("index.pluto"):split("\n$haltcompiler"):back():strip()) --> Anything after the $haltcompiler statement is not processed as Pluto source code.
$haltcompiler
Anything after the $haltcompiler statement is not processed as Pluto source code.