@algosail/logic

Logical connectives and boolean control-flow combinators.

Contents

• and
• or
• not
• complement
• boolean_
• ifElse
• when
• unless

and

and :: Boolean -> Boolean -> Boolean

Logical conjunction.

and(true)(true) // => true
and(true)(false) // => false
and(false)(false) // => false

or

or :: Boolean -> Boolean -> Boolean

Logical disjunction.

or(false)(true) // => true
or(false)(false) // => false
or(true)(true) // => true

not

not :: Boolean -> Boolean

Logical negation.

not(true) // => false
not(false) // => true

complement

complement :: (a -> Boolean) -> a -> Boolean

Returns a predicate that negates the original. Useful for building inverse filters.

const isEven = (x) => x % 2 === 0
const isOdd = complement(isEven)

isOdd(3) // => true
isOdd(4) // => false
  [(1, 2, 3, 4)].filter(complement(isEven)) // => [1, 3]

boolean_

boolean_ :: a -> a -> Boolean -> a

Case analysis on a boolean — returns onFalse or onTrue.

boolean_('no')('yes')(true) // => 'yes'
boolean_('no')('yes')(false) // => 'no'
boolean_(0)(1)(true) // => 1

ifElse

ifElse :: (a -> Boolean) -> (a -> b) -> (a -> b) -> a -> b

Applies f if the predicate holds, g otherwise. Both branches receive the original value.

ifElse((x) => x > 0)((x) => x)((x) => -x)(-3) // => 3   (abs)
ifElse((x) => x > 0)((x) => x)((x) => -x)(5) // => 5
ifElse(Array.isArray)((arr) => arr.length)((s) => s.length)([1, 2, 3]) // => 3
ifElse(Array.isArray)((arr) => arr.length)((s) => s.length)('hello') // => 5

when

when :: (a -> Boolean) -> (a -> a) -> a -> a

Applies f only when the predicate holds; returns the value unchanged otherwise.

when((x) => x < 0)((x) => 0)(-5) // => 0
when((x) => x < 0)((x) => 0)(5) // => 5

// Guard against empty strings
when((s) => s.length === 0)(() => 'default')('') // => 'default'
when((s) => s.length === 0)(() => 'default')('hi') // => 'hi'

unless

unless :: (a -> Boolean) -> (a -> a) -> a -> a

Applies f only when the predicate does not hold. The inverse of when.

unless((x) => x > 0)((x) => -x)(-3) // => 3   (negated because not positive)
unless((x) => x > 0)((x) => -x)(5) // => 5   (left unchanged)

// Ensure string is trimmed unless it's already clean
unless((s) => s === s.trim())((s) => s.trim())('  hi  ') // => 'hi'