YIELD

The YIELD keyword creates anonymous functions (also known as lambdas). It connects parameters declared with GIVEN to an expression body, without naming the function.

Syntax

GIVEN parameters YIELD expression

Basic Usage

Single Parameter Lambda

GIVEN x YIELD x + 1

This creates a function that takes a number and returns that number plus one.

Multiple Parameters

Parameters can be separated by commas:

GIVEN x, y YIELD x + y

Or using layout (one per line):

GIVEN a
      b
YIELD a * b

Typed Parameters

You can specify types for lambda parameters:

GIVEN n IS A NUMBER YIELD n * 2

With multiple typed parameters:

GIVEN name IS A STRING
      age IS A NUMBER
YIELD name

Common Use Cases

With Higher-Order Functions

Lambdas are most commonly used with higher-order functions from the prelude:

Universal quantification (all elements satisfy condition):

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

Existential quantification (some element satisfies condition):

GIVEN numbers IS A LIST OF NUMBER
GIVETH A BOOLEAN
`has negative` MEANS any (GIVEN n YIELD n < 0) numbers

Transforming lists (map):

GIVEN numbers IS A LIST OF NUMBER
GIVETH A LIST OF NUMBER
`doubled` MEANS map (GIVEN n YIELD n * 2) numbers

Filtering lists:

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

With Record Types

Access record fields in lambda bodies:

DECLARE Person HAS
  name IS A STRING
  age IS A NUMBER

GIVEN people IS A LIST OF Person
GIVETH A BOOLEAN
`all adults` MEANS all (GIVEN p YIELD p's age >= 18) people

Function Composition

Return functions from functions:

GIVEN a IS A TYPE
      b IS A TYPE
      c IS A TYPE
      f IS A FUNCTION FROM b TO c
      g IS A FUNCTION FROM a TO b
GIVETH A FUNCTION FROM a TO c
compose f g MEANS GIVEN x IS AN a YIELD f (g x)

Parentheses Requirements

When passing lambdas to functions, you typically need parentheses around:

The lambda expression itself
The list being processed (especially with 's accessor)

-- ✅ Correct: parentheses around lambda and list
all (GIVEN p YIELD p's age >= 18) (people's members)

-- ❌ Wrong: missing parentheses around list
all (GIVEN p YIELD p's age >= 18) people's members

YIELD vs MEANS

Aspect	YIELD	MEANS
Creates	Anonymous function (lambda)	Named function
Used with	GIVEN (inline)	GIVEN (top-level) or DECIDE
Function name	None	Required
Typical use	Passing to higher-order functions	Defining reusable functions

Named function with MEANS:

GIVEN n IS A NUMBER
GIVETH A NUMBER
double n MEANS n * 2

Anonymous function with YIELD:

DECIDE double IS GIVEN n YIELD n * 2

Both create equivalent functions, but YIELD is preferred for inline use.

Full Example

Example:

-- YIELD keyword examples: Anonymous functions (lambdas)

IMPORT prelude


-- BASIC LAMBDA SYNTAX


-- Simple single-parameter lambda
DECIDE `add one` IS GIVEN x YIELD x + 1

-- Multiple parameters (comma-separated)
DECIDE `add together` IS GIVEN x, y YIELD x + y

-- Multiple parameters (layout-based)
DECIDE `multiply values` IS
  GIVEN a
        b
  YIELD a * b


-- TYPED LAMBDAS


-- Single typed parameter
DECIDE `double number` IS GIVEN n IS A NUMBER YIELD n * 2

-- Multiple typed parameters
DECIDE `format pair` IS
  GIVEN personName IS A STRING
        personAge IS A NUMBER
  YIELD personName


-- LAMBDAS WITH HIGHER-ORDER FUNCTIONS


-- Using lambda with `all` (universal quantifier)
GIVEN numbers IS A LIST OF NUMBER
GIVETH A BOOLEAN
`all positive` MEANS all (GIVEN n YIELD n > 0) numbers

-- Using lambda with `any` (existential quantifier)
GIVEN numbers IS A LIST OF NUMBER
GIVETH A BOOLEAN
`has negative` MEANS any (GIVEN n YIELD n < 0) numbers

-- Using lambda with `map`
GIVEN numbers IS A LIST OF NUMBER
GIVETH A LIST OF NUMBER
`doubled list` MEANS map (GIVEN n YIELD n * 2) numbers

-- Using lambda with `filter`
GIVEN numbers IS A LIST OF NUMBER
GIVETH A LIST OF NUMBER
`only positive` MEANS filter (GIVEN n YIELD n > 0) numbers


-- COMPLEX LAMBDA EXAMPLES


-- Record field access in lambda
DECLARE Person HAS
  name IS A STRING
  age IS A NUMBER

GIVEN people IS A LIST OF Person
GIVETH A BOOLEAN
`all adults` MEANS all (GIVEN p YIELD p's age >= 18) people

-- Nested lambdas (currying)
GIVEN predicate IS A FUNCTION FROM NUMBER TO BOOLEAN
GIVETH A FUNCTION FROM NUMBER TO BOOLEAN
negate predicate MEANS GIVEN x YIELD NOT predicate x

-- Lambda returning a function
GIVEN a IS A TYPE
      b IS A TYPE
      c IS A TYPE
      f IS A FUNCTION FROM b TO c
      g IS A FUNCTION FROM a TO b
GIVETH A FUNCTION FROM a TO c
compose f g MEANS GIVEN x IS AN a YIELD f (g x)


-- EVALUATION TESTS


#EVAL `add one` 5
#EVAL `add together` 3 7
#EVAL `double number` 21