Lambda

ReQL Lambda functions can be used for computations on sets of values.

Lambda functions take two arguments: a lambda expression and a set.

Lambda functions evaluate the lambda expression on every element of the set. The lambda expression can be any valid ReQL expression, and therefore can contain for example item properties or functions.

Name	Description	Example
map	Applies the lambda expression to every member of a set	`map(lambda 'x': 2*'x', {1, 2, 3}) == {2, 4, 6}`
select	Returns only those values of input set for which the lambda expression returns true	`select(lambda 'x': 'x' > 5, {10, 2, 13, 1}) == {10, 13}`
exists	Returns true if lambda expression is satisfied for at least one element of the input set	`exists(lambda 'x': 'x' > 5, {2, 13, 4})`

Map

Map applies the expression to every element and returns the result as set.

Definition

map(lambda 'x': EXPRESSION, input_set)

Argument	Type
lambda expression	`lambda expression`
input_set	`set`

Examples

All the following expressions result in true:

// Multiplies every element of the input set by two
map(lambda 'x': 2\*'x', {1, 2, 3}) == {2, 4, 6}

// Converts every element from the input set to number
map(lambda 'x': number('x'), {"1", "2", "3"}) == {1, 2, 3}

// Map applied on empty set is empty set
map(lambda 'x': 'x'*'x', {}) == {}

Select

Select returns only those values of input set for which the lambda expression returns true. The lambda expression have to return a boolean.

Definition

select(lambda 'x': EXPRESSION, input_set)

Argument	Type
lambda expression	`lambda expression` returning boolean
input_set	`set`

Examples

All the following expressions result in true:

// Selects only the values that are greater than five
select(lambda 'x': 'x'>5, {10, 2, 13, 8, 3, 4, 7}) == {10, 13, 8, 7}

// ''true'' is satisfied for every element
select(lambda 'x': true , {1, "a", {}}) == {1, "a", {}}

// Selects only the strings, that are lowercase
select(lambda 'x': lower('x') == 'x', {"HELLO", "hello", "Hello", "a"}) == {"hello", "a"}

// No element represents a number with value less then 100
select(lambda 'x': number('x')<100 , {"125", "123", "251"}) == {}

Exists

Exists returns true if lambda expression returns true for at least one element of the input set. Otherwise returns false.

exists(lambda 'x': ...) can be though as shorthand for size(select(lambda 'x': ...)) > 0.

Definition

exists(lambda 'x': EXPRESSION, input_set)

Argument	Type
lambda expression	`lambda expression` returning boolean
input_set	`set`

Examples

All the following expressions result in true:

// Returns true, as 13 > 5
exists(lambda 'x': 'x'>5, {2, 13, 4})

// None of the elements is larger than 5
exists(lambda 'x': 'x'>5, {2, 1, 4}) == false

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