sanctuary-maybe
The Maybe type represents optional values: a value of type Maybe a is
either Nothing (the empty value) or a Just whose value is of type a.
Maybe a satisfies the following Fantasy Land specifications:
> const Useless = require ('sanctuary-useless')
> const isTypeClass = x =>
. type (x) === 'sanctuary-type-classes/TypeClass@1'
> S.map (k => k + ' '.repeat (16 - k.length) +
. (Z[k].test (Just (Useless)) ? '\u2705 ' :
. Z[k].test (Nothing) ? '\u2705 * ' :
. '\u274C '))
. (S.keys (S.unchecked.filter (isTypeClass) (Z)))
[ 'Setoid ✅ * ',
. 'Ord ✅ * ',
. 'Semigroupoid ❌ ',
. 'Category ❌ ',
. 'Semigroup ✅ * ',
. 'Monoid ✅ * ',
. 'Group ❌ ',
. 'Filterable ✅ ',
. 'Functor ✅ ',
. 'Bifunctor ❌ ',
. 'Profunctor ❌ ',
. 'Apply ✅ ',
. 'Applicative ✅ ',
. 'Chain ✅ ',
. 'ChainRec ✅ ',
. 'Monad ✅ ',
. 'Alt ✅ ',
. 'Plus ✅ ',
. 'Alternative ✅ ',
. 'Foldable ✅ ',
. 'Traversable ✅ ',
. 'Extend ✅ ',
. 'Comonad ❌ ',
. 'Contravariant ❌ ' ]
Maybe type representative.
The empty value of type Maybe a.
> Nothing
Nothing
Constructs a value of type Maybe a from a value of type a.
> Just (42)
Just (42)
empty (Maybe) is equivalent to Nothing
> S.empty (Maybe)
Nothing
of (Maybe) (x) is equivalent to Just (x)
> S.of (Maybe) (42)
Just (42)
> Z.chainRec (
. Maybe,
. (next, done, x) =>
. x <= 1 ? Nothing : Just (x >= 1000 ? done (x) : next (x * x)),
. 1
. )
Nothing
> Z.chainRec (
. Maybe,
. (next, done, x) =>
. x <= 1 ? Nothing : Just (x >= 1000 ? done (x) : next (x * x)),
. 2
. )
Just (65536)
zero (Maybe) is equivalent to Nothing
> S.zero (Maybe)
Nothing
show (Nothing) is equivalent to 'Nothing'show (Just (x)) is equivalent to 'Just (' + show (x) + ')'
> show (Nothing)
'Nothing'
> show (Just (['foo', 'bar', 'baz']))
'Just (["foo", "bar", "baz"])'
Nothing is equal to NothingJust (x) is equal to Just (y) iff x is equal to y
according to Z.equalsNothing is never equal to Just (x)
> S.equals (Nothing) (Nothing)
true
> S.equals (Just ([1, 2, 3])) (Just ([1, 2, 3]))
true
> S.equals (Just ([1, 2, 3])) (Just ([3, 2, 1]))
false
> S.equals (Just ([1, 2, 3])) (Nothing)
false
Nothing is (less than or) equal to NothingJust (x) is less than or equal to Just (y) iff x is less
than or equal to y according to Z.lteNothing is always less than Just (x)
> S.filter (S.lte (Nothing)) ([Nothing, Just (0), Just (1), Just (2)])
[Nothing]
> S.filter (S.lte (Just (1))) ([Nothing, Just (0), Just (1), Just (2)])
[Nothing, Just (0), Just (1)]
concat (Nothing) (Nothing) is equivalent to Nothingconcat (Just (x)) (Just (y)) is equivalent to
Just (concat (x) (y))concat (Nothing) (Just (x)) is equivalent to Just (x)concat (Just (x)) (Nothing) is equivalent to Just (x)
> S.concat (Nothing) (Nothing)
Nothing
> S.concat (Just ([1, 2, 3])) (Just ([4, 5, 6]))
Just ([1, 2, 3, 4, 5, 6])
> S.concat (Nothing) (Just ([1, 2, 3]))
Just ([1, 2, 3])
> S.concat (Just ([1, 2, 3])) (Nothing)
Just ([1, 2, 3])
filter (p) (Nothing) is equivalent to Nothingfilter (p) (Just (x)) is equivalent to p (x) ? Just (x) : Nothing
> S.filter (isFinite) (Nothing)
Nothing
> S.filter (isFinite) (Just (Infinity))
Nothing
> S.filter (isFinite) (Just (Number.MAX_SAFE_INTEGER))
Just (9007199254740991)
map (f) (Nothing) is equivalent to Nothingmap (f) (Just (x)) is equivalent to Just (f (x))
> S.map (Math.sqrt) (Nothing)
Nothing
> S.map (Math.sqrt) (Just (9))
Just (3)
ap (Nothing) (Nothing) is equivalent to Nothingap (Nothing) (Just (x)) is equivalent to Nothingap (Just (f)) (Nothing) is equivalent to Nothingap (Just (f)) (Just (x)) is equivalent to Just (f (x))
> S.ap (Nothing) (Nothing)
Nothing
> S.ap (Nothing) (Just (9))
Nothing
> S.ap (Just (Math.sqrt)) (Nothing)
Nothing
> S.ap (Just (Math.sqrt)) (Just (9))
Just (3)
chain (f) (Nothing) is equivalent to Nothingchain (f) (Just (x)) is equivalent to f (x)
> const head = xs => xs.length === 0 ? Nothing : Just (xs[0])
> S.chain (head) (Nothing)
Nothing
> S.chain (head) (Just ([]))
Nothing
> S.chain (head) (Just (['foo', 'bar', 'baz']))
Just ('foo')
alt (Nothing) (Nothing) is equivalent to Nothingalt (Just (x)) (Nothing) is equivalent to Just (x)alt (Nothing) (Just (x)) is equivalent to Just (x)alt (Just (y)) (Just (x)) is equivalent to Just (x)
> S.alt (Nothing) (Nothing)
Nothing
> S.alt (Just (1)) (Nothing)
Just (1)
> S.alt (Nothing) (Just (2))
Just (2)
> S.alt (Just (4)) (Just (3))
Just (3)
reduce (f) (x) (Nothing) is equivalent to xreduce (f) (x) (Just (y)) is equivalent to f (x) (y)
> S.reduce (S.concat) ('abc') (Nothing)
'abc'
> S.reduce (S.concat) ('abc') (Just ('xyz'))
'abcxyz'
traverse (A) (f) (Nothing) is equivalent to of (A) (Nothing)traverse (A) (f) (Just (x)) is equivalent to map (Just) (f (x))
> S.traverse (Array) (S.words) (Nothing)
[Nothing]
> S.traverse (Array) (S.words) (Just ('foo bar baz'))
[Just ('foo'), Just ('bar'), Just ('baz')]
extend (f) (Nothing) is equivalent to Nothingextend (f) (Just (x)) is equivalent to Just (f (Just (x)))
> S.extend (S.reduce (S.add) (1)) (Nothing)
Nothing
> S.extend (S.reduce (S.add) (1)) (Just (99))
Just (100)
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