sweet.js でのアレ

README.md

sjs --module ./mod.js --output fn.built.js fn.js
sjs --module ./mod.js --output fn2.built.js fn2.js
sjs --module ./mod.js --output fn3.built.js fn3.js

fn.js

fn x y { x };

fn2.js

square = fn x { x * x };
cube   = fn x { square(x) * x };

fn3.js

fill = fn container [liquid = "coffee"] {
  "Filling the #{container} with #{liquid}..."
};

mod.js

macro setdefault {
  rule { $var = $init } => { if ($var == null) { $var = $init } }
}

macro setdefaults {
  rule { {                          } } => {}
  rule { { $var = $init $repeat ... } } => { setdefault $var = $init ; setdefaults { $repeat ... } }
}

macro setdefaults1 {
  rule { {                =                  } } => {}
  rule { { $var $vars ... = $init $inits ... } } => { setdefault $var = $init ; setdefaults1 { $vars ... = $inits ... } }
}

macro fn {
  rule { $params ... [$($option = $init) ...] { $body ... } } => { (function ($params ... $option ...) { setdefaults1 { $option ... = $init ... } ; return ($body ...) }) }
  rule { $params ...                          { $body ... } } => { (function ($params ...) { return ($body ...) }) }
}

macro when {
  rule {
    $cond { $body ... }
  } => {
    {
      if ($cond) { $body ... }
    }
  }
}

export fn;
export when;