I am trying this from scratch learning about Contravariants and deeper knowledge of Sanctuary. The code "works" but again I don't have the types exactly right.
Here is the Contravariant
const {contramap: contramapFl, extract } = require('fantasy-land');
const getInstance = (self, constructor) =>
(self instanceof constructor) ?
self :
Object.create(constructor.prototype) ;
// Contra a ~> g: a -> ?
const Contra = function(g){
const self = getInstance(this, Contra)
// :: F a ~> b -> a -> F b [ b -> a -> ? ]
self[contramapFl] = f => Contra( x => g(f(x)) )
self[extract] = g
self['@@type'] = 'fs-javascript/contra'
return Object.freeze(self)
}
// UPDATE adding type to constructor
Contra['@@type'] = 'fs-javascript/contra'
And my attempt to get the types right
const $ = require('sanctuary-def');
const type = require('sanctuary-type-identifiers');
const Z = require('sanctuary-type-classes') ;
const isContra = x => type (x) === 'fs-javascript/contra'
const ContraType = $.UnaryType(
'fs-javascript/contra',
'http://example.com/fs-javascript#Contra',
isContra,
x => [x[extract]])($.Unknown)
Then my test
const {create, env} = require('sanctuary');
const {contramap} = create({checkTypes: true, env: env.concat(ContraType) });
const isEven = Contra(x => x % 2 === 0) ;
console.log(Z.Contravariant.test(isEven)) // => true
const isLengthEvenContra = contramap(y => y.length, isEven)
const isStringLengthEven = isLengthEvenContra[extract]
console.log(isStringLengthEven("asw")) //=> ERROR
TypeError: Type-variable constraint violation contramap :: Contravariant f => (b -> a) -> f a -> f b ^ 1 1) "fs-javascript/contra" :: String f => Contra( x => g(f(x)) ) :: Function, (c -> d) x => x % 2 === 0 :: Function, (c -> d) Since there is no type of which all the above values are members, the type-variable constraint has been violated.
If I disable the type checking then it works as expected, so logically it appears to be stitched together properly. I defined my own version of contramap
const def = $.create({ checkTypes: true, env: $.env.concat(ContraType) });
const contramap2 =
def('contramap2', {}, [$.Unknown, ContraType, ContraType],
(f, x) => {
const z = x[contramapFl](f)
return z
}
)
I then rerun the test:
const isEven = Contra(x => x % 2 === 0) ;
console.log(Z.Contravariant.test(isEven)) // => true
const isLengthEvenContra = contramap2(y => y.length, isEven)
const isStringLengthEven = isLengthEvenContra[extract]
console.log(isStringLengthEven("asw")) //=> false
So withstanding the discussion as to whether the contravaiant functor is the best approach to this problem (learning exercise), the question is how, when defining my own implementation of a contravariant, can I use sanctuary's contramap function with the type checking enabled.
after updating by adding the code:
Contra['@@type'] = 'fs-javascript/contra'
changed the error to:
TypeError: Type-variable constraint violation contramap :: Contravariant f => (b -> a) -> f a -> f b ^ ^ 1 2 1) 3 :: Number, FiniteNumber, NonZeroFiniteNumber, Integer, NonNegativeInteger, ValidNumber 2) x => x % 2 === 0 :: Function, (c -> d) Since there is no type of which all the above values are members, the type-variable constraint has been violated.
// Contra (Integer -> Boolean)
const isEven = Contra(x => x % 2 === 0) ;
// String -> Integer
const strLength = y => y.length
// I Think: Contra (String -> (Integer -> Boolean))
const isLengthEvenContra = contramap(strLength, isEven)
// (String -> (Integer -> Boolean))
const isStringLengthEven = isLengthEvenContra[extract]
My understanding of the contravariant functor was that it pre-composed the function within it, with the function passed in via contramap
. So if the contravariant contained the function f
and it is contramap
with g
it returns a new contravariant functor wrapping x = g(f(x))
Have i misunderstood this (too)
You're composing functions. Sanctuary defines
fantasy-land/map
andfantasy-land/contramap
forFunction a b
, so there's no need for a Contra wrapping type.