Functions Reference

This section documents the keywords used to define functions and computations in L4. Functions represent individual rules.

Overview

L4 functions are defined using a combination of keywords that specify parameters, return types, and the function body. The typical structure is:

GIVEN parameter IS A Type      -- Parameters (optional)
GIVETH A ReturnType            -- Return type (optional)
DECIDE name MEANS expression   -- Definition

Keywords

Keyword	Purpose	Required
GIVEN	Declares function parameters with types	For functions with inputs
GIVETH / GIVES	Specifies the return type	Optional (can be inferred)
DECIDE	Defines a named value or function	Yes (or use bare MEANS)
MEANS	Connects the function name to its body	Yes
YIELD	Creates anonymous functions (lambdas)	For inline functions
WHERE	Adds local definitions after an expression	Optional
LET	Adds local definitions before an expression	Optional
AKA	Creates aliases for definitions	Optional

Quick Examples

Simple Function

GIVEN x IS A NUMBER
double x MEANS x TIMES 2

Fully Annotated Function

GIVEN n IS A NUMBER
GIVETH A NUMBER
DECIDE factorial n MEANS
  IF n EQUALS 0
  THEN 1
  ELSE n TIMES factorial (n MINUS 1)

With Local Definitions

circleArea radius MEANS
  pi TIMES radius TIMES radius
  WHERE
    pi MEANS 3.14159

With Aliases

`total cost` AKA `final price` MEANS
  basePrice PLUS tax

Anonymous Functions (Lambdas)

Use GIVEN ... YIELD for inline functions:

GIVEN numbers IS A LIST OF NUMBER
`all positive` MEANS all (GIVEN n YIELD n > 0) numbers

Function Definition Patterns

Named Constants

No parameters needed:

DECIDE pi IS 3.14159
DECIDE greeting IS "Hello, World!"

Single Parameter

GIVEN x IS A NUMBER
square x MEANS x TIMES x

Multiple Parameters

GIVEN a IS A NUMBER
      b IS A NUMBER
add a b MEANS a PLUS b

Boolean Decisions

Use DECIDE ... IF for boolean-returning functions:

GIVEN age IS A NUMBER
DECIDE `is adult` IF age >= 18

Local Bindings

Two styles for introducing local definitions:

WHERE (definitions after)

result MEANS x PLUS y
  WHERE
    x MEANS 10
    y MEANS 20

LET (definitions before)

result MEANS
  LET x IS 10, y IS 20
  IN x PLUS y