Composing Predicates

composing-predicates.js

const filterText = this.props.filterText;
const inStockOnly = this.props.inStockOnly;
​
// K-combinator, used to create "always true" predicates
const K = k => () => k;
// Curried Array.prototype.filter
const filter = pr => xs => xs.filter(pr);
// Predicate generators
const byStock = () => ({stock}) => stock !== false;
const byName = text => ({name}) => name.indexOf(text) !== -1;
​
// Wraps a predicate into a function that will take a boolean flag.
// If this flag is true, the predicate is returned. Otherwise, an always-true predicate is returned.
const conditionalPredicate = pr => flag => flag ? pr : K(true);
​
const byStockCond = conditionalPredicate(byStock());
const byNameCond = conditionalPredicate(byName(filterText));
​
// Joins predicates with logical conjunction (&&)
const joinPredicates = (...prs) => x => prs.every(pr => pr(x));
​
const buildFilter = (byStock, byName) => filter(joinPredicates(byStockCond(byStock), byNameCond(byName)));

ProductTable.jsx

const Just = v => ({
  isJust: true,
  value: v
});
const Nothing = { isJust: false };
const last = xs => xs.length > 0 ? Just(xs[xs.length-1]) : Nothing;
const catMaybes = xs => xs.reduce((newList, m) => m.isJust && newList.push(m.value), []);
const flatten = () => this.reduce((newList, l) => newList.concat(l), []);
const compose = f => g => x => f(g(x));
// ...

class ProductTable extends React.Component {
  render() {
    const filterText = this.props.filterText;
    const inStockOnly = this.props.inStockOnly;
    
    const rows = compose (catMaybes) (flatten)
    (this.props.product
      .filter(p => p.name.includes(filterText))
      .filter(p => inStockOnly && !product.stocked)
      .reduce((rows, p) => rows.push([last(rows), p]), [])
      .map(([prev, curr]) => {
        const lastCategory = prev.isJust ? prev.value.category : '';
        return [
          curr.category !== lastCategory ?
          Just(<ProductCategoryRow
            category={curr.category}
            key={curr.category} />
          ) : Nothing,
          Just(<ProductRow
            product={product}
            key={product.name}
          />)
        ];
      }));

    return (
      <table>
        <thead>
          <tr>
            <th>Name</th>
            <th>Price</th>
          </tr>
        </thead>
        <tbody>{rows}</tbody>
      </table>
    );
  }