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Functions

A function takes inputs and computes a result from them. In L4 each rule is a function: GIVEN names and types its inputs, GIVETH states the type of its result, and DECIDE or MEANS gives the expression that computes it. A function without inputs is a named value.

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

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  • DECIDE: define a value or a function, including DECIDE … IF and functions written as several clauses
  • GIVEN and GIVETH: declare the inputs and the result type
  • MEANS: define without DECIDE
  • LET and WHERE: local definitions before or after an expression
  • YIELD: functions without a name
  • AKA: more than one name for the same function, value, type or section

Mixfix functions

A function's name does not have to come first. In a mixfix function, the GIVEN inputs are placed between the words of the name, so a call reads like a sentence:

GIVEN mom IS A STRING
      dad IS A STRING
      kid IS A STRING
GIVETH A STRING
mom and dad `have a baby named` kid MEANS
    CONCAT "mother: ", mom, ", father: ", dad, ", child: ", kid

#EVAL "alice" and "bob" `have a baby named` "carol"
  • A call uses the same word order as the definition.
  • Inputs can come before, between or after the words, so a function can be infix (applicant `is entitled to` benefit) or postfix (n squared).
  • A word of the name can be a quoted identifier with spaces, such as `is entitled to`, so the words can be legal phrases.
  • The words that are not inputs act as keywords of that function. When a call mixes inputs and words, L4 finds the function by those words.

An infix function can also be a later operand of a built-in operator such as AND, OR, PLUS or EQUALS:

GIVEN a IS A NUMBER
      b IS A NUMBER
GIVETH A BOOLEAN
a `exceeds by at least` b MEANS a AT LEAST b

#EVAL TRUE AND 5 `exceeds by at least` 3
  • The call needs no parentheses, because it binds tighter than the infix operators in the precedence table. See Precedence.
  • All the words of the call must be on the operand's own line.
  • To spread a call over several lines, put it in parentheses and start a new line after the opening parenthesis.

A mixfix function cannot be written as several clauses; use CONSIDER in its body.

When to use mixfix, with worked examples: Make rules read like law.