Ramda: Split a list to segments - ramda.js

Question

If we have a list such as:
[
{
type: 'a',
},
{
type: 'a',
},
{
type: 'b',
},
{
type: 'a',
}
]
... and we want to segment it to create a list, such that the new list is made up of each segment of the initial list, here split by type, looking like:
[
[
{
type: 'a',
},
{
type: 'a',
},
],
[
{
type: 'b',
},
],
[
{
type: 'a',
}
]
]
I'd like to create a general purpose 'segmenting' function, which takes a function to compare two items, and determine whether or not a new segment is required. Here, the 'segmenter' for that function simply compares type.
I can write that in vanilla javascript, but is there a good way to do this with Ramda?
const data = [
{
type: 'a',
},
{
type: 'a',
},
{
type: 'b',
},
{
type: 'a',
}
];
const segmentBy = segmenter => items => {
const segmentReducer = (prev = [], curr) => {
let lastSegment = [];
let lastItem = null;
try {
lastSegment = prev[prev.length - 1];
lastItem = lastSegment[lastSegment.length - 1];
} catch (e) {
return [...prev, [curr]];
}
const requiresNewSegment = segmenter(lastItem, curr);
if (requiresNewSegment) {
return [...prev, [curr]];
}
return [...prev.slice(0, prev.length - 1), [...lastSegment, curr]];
};
return items.reduce(segmentReducer, []);
};
const segmentByType = segmentBy((a, b) => a.type !== b.type);
const segments = segmentByType(data);
console.dir(segments);

Answer 1

With Ramda you can use R.groupWith:
Takes a list and returns a list of lists where each sublist's elements
are all satisfied pairwise comparison according to the provided
function. Only adjacent elements are passed to the comparison
function.
const data = [{"type":"a"},{"type":"a"},{"type":"b"},{"type":"a"}];
const segmentByType = R.groupWith(R.eqBy(R.prop('type')));
const segments = segmentByType(data);
console.dir(segments);
<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.26.1/ramda.js"></script>
In vanilla, the main problem is when to add a new subarray to the accumulator. You need to add another subarray when it's the 1st item, or if segmenter returns true.
const data = [{"type":"a"},{"type":"a"},{"type":"b"},{"type":"a"}];
const segmentBy = segmenter => items =>
items.reduce((r, item, i, arr) => {
if(i === 0 || segmenter(item, arr[i - 1])) r.push([]);
r[r.length - 1].push(item);
return r;
}, []);
const segmentByType = segmentBy((a, b) => a.type !== b.type);
const segments = segmentByType(data);
console.dir(segments);