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September 25, 2017 17:24
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'use strict'; | |
// | |
// # Option | |
// | |
// Option a = Some a + None | |
// | |
// The option type encodes the presence and absence of a value. The | |
// `Some` constructor represents a value and `None` represents the | |
// absence. | |
// | |
// * `ap(s)` - Applicative ap(ply) | |
// * `chain(f)` - Monadic flatMap/bind | |
// * `concat(s, plus)` - Semigroup concat | |
// * `fold(a, b)` - Applies `a` to value if `Some` or defaults to `b` | |
// * `orElse(a)` - Default value for `None` | |
// * `getOrElse(a)` - Default value for `None` | |
// * `map(f)` - Functor map | |
// | |
const {tagged, taggedSum} = require('daggy'); | |
const {constant, identity} = require('fantasy-combinators'); | |
const {of, empty, ap, map, chain, concat, sequence} = require('fantasy-land'); | |
const Option = taggedSum({ | |
Some: ['x'], | |
None: [] | |
}); | |
// Methods | |
Option.prototype.fold = function(f, g) { | |
return this.cata({ | |
Some: f, | |
None: g | |
}); | |
}; | |
Option[of] = Option.Some; | |
Option[empty] = () => Option.None; | |
Option.prototype.orElse = function(x) { | |
return this.fold( | |
Option.Some, | |
constant(x) | |
); | |
}; | |
Option.prototype.getOrElse = function(x) { | |
return this.fold( | |
identity, | |
constant(x) | |
); | |
}; | |
Option.prototype[chain] = function(f) { | |
return this.fold( | |
(a) => f(a), | |
constant(Option.None) | |
); | |
}; | |
Option.prototype[concat] = function(x) { | |
return this[chain]((a) => { | |
return x[map]((b) => a[concat](b)); | |
}); | |
}; | |
// Derived | |
Option.prototype[map] = function(f) { | |
return this[chain]((a) => Option[of](f(a))); | |
}; | |
Option.prototype[ap] = function(a) { | |
return this[chain]((f) => a[map](f)); | |
}; | |
Option.prototype[sequence] = function(p) { | |
return this.traverse(identity, p); | |
}; | |
Option.prototype.traverse = function(f, p) { | |
return this.cata({ | |
Some: (x) => f(x)[map](Option[of]), | |
None: () => p[of](Option.None) | |
}); | |
}; | |
// Transformer | |
Option.OptionT = (M) => { | |
const OptionT = tagged('run'); | |
OptionT.prototype.fold = function(f, g) { | |
return this.run[chain]((o) => M[of](o.fold(f, g))); | |
}; | |
OptionT[of] = (x) => OptionT(M[of](Option.Some(x))); | |
OptionT.prototype.orElse = function(b) { | |
return OptionT(this.run[chain]((a) => { | |
return a.fold( | |
(x) => M[of](a), | |
() => b.run | |
); | |
})); | |
}; | |
OptionT.prototype.getOrElse = function(x) { | |
return this.run[chain]((o) => M[of](o.getOrElse(x))); | |
}; | |
OptionT.prototype[chain] = function(f) { | |
return OptionT(this.run[chain]((o) => { | |
return o.fold( | |
(a) => f(a).run, | |
() => M[of](Option.None) | |
); | |
})); | |
}; | |
OptionT.prototype[map] = function(f) { | |
return this[chain]((a) => OptionT[of](f(a))); | |
}; | |
OptionT.prototype[ap] = function(a) { | |
return this[chain]((f) => a[map](f)); | |
}; | |
return OptionT; | |
}; | |
module.exports = Option; |
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