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specify location of ‘empty’ method #162

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specify location of ‘empty’ method #162

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5 changes: 2 additions & 3 deletions README.md
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Expand Up @@ -138,11 +138,10 @@ the Semigroup specification.
empty :: Monoid m => () -> m
```

A value which has a Monoid must provide an `empty` method on itself or
its `constructor` object. The `empty` method takes no arguments:
A value which has a Monoid must provide an `empty` method. The `empty`
method takes no arguments:

m.empty()
m.constructor.empty()

1. `empty` must return a value of the same Monoid

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5 changes: 2 additions & 3 deletions id.js
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Expand Up @@ -15,10 +15,9 @@ Id.prototype[fl.concat] = function(b) {
};

// Monoid (value must also be a Monoid)
Id[fl.empty] = function() {
return new Id(this.value[fl.empty] ? this.value[fl.empty]() : this.value.constructor[fl.empty]());
Id.prototype[fl.empty] = function() {
return new Id(this.value[fl.empty]());
};
Id.prototype[fl.empty] = Id[fl.empty];

// Foldable
Id.prototype[fl.reduce] = function(f, acc) {
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7 changes: 3 additions & 4 deletions id_test.js
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Expand Up @@ -22,9 +22,8 @@ const Id = require('./id');
// Special type of sum for the type of string.
const Sum = tagged('v');
Sum[of] = (x) => Sum(x);
Sum[empty] = () => Sum('');
Sum.prototype[of] = Sum[of];
Sum.prototype[empty] = Sum[empty];
Sum.prototype[empty] = () => Sum('');
Sum.prototype[map] = function(f) {
return Sum(f(this.v));
};
Expand Down Expand Up @@ -90,8 +89,8 @@ exports.monad = {
};

exports.monoid = {
leftIdentity: test((x) => monoid.leftIdentity(Id[of](Sum[empty]()))(equality)(Sum[of](x))),
rightIdentity: test((x) => monoid.rightIdentity(Id[of](Sum[empty]()))(equality)(Sum[of](x)))
leftIdentity: test((x) => monoid.leftIdentity(Id[of](Sum.prototype[empty]()))(equality)(Sum[of](x))),
rightIdentity: test((x) => monoid.rightIdentity(Id[of](Sum.prototype[empty]()))(equality)(Sum[of](x)))
};

// Semigroup tests are broken otherwise for this.
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