lambda.md

Lambda Expressions

also nicknamed do expressions

Lambda expressions in Lunar is really just an anonymous function. Rather than typing the verbose variant function(params) return ... end, you can greatly simplify this with |params| ....

There are two forms of the lambda expressions. First one is a concise body, and the other one is a block body.

Concise body

The syntax of a concise body lambda expressions is...

local a = |param1, param2, ...| param1 + param2

-- Which is the equivalent of...
local a = function(param1, param2, ...)
  return param1 + param2
end

It is also legal to have parameterless lambda expressions.

local b = || 1

-- Which is the equivalent of...
local b = function()
  return 1
end

A concise body implicitly returns the expression, so you do not need to type |...| return ..., never mind the fact that it's illegal syntax.

If you can assign your expression to local something, then it is legal to use that same expression. The above example would only be legal if you could do local something = return ..., which you cant so it's therefore illegal.

Block body

NOTE: This is currently a candidate for a different syntax: do |...| code end.

NOTE: You can still use do end statement normally, as this is actually completely separate from a do end statement.

Block-body lambda expressions are similar to the concise-body variant, with a few key differences.

One difference is: for zero parameters, omit it altogether, and for one or more parameters, add |param1| just before the do keyword.

Another difference is that they allow multiple statements...

local a = do
  local a = 1
  local b = 2

  return a + b -- and the return statement is not implicit!
end

-- Which is the same as...
local a = function()
  local a = 1
  local b = 2

  return a + b
end

And for block-body lambda expressions with parameters...

local b = |param1, param2, ...| do
  return param1 + param2
end

-- Which is the same as...
local b = function(param1, param2, ...)
  return param1 + param2
end