# fold

Source: https://taxilang.org/docs/language/stdlib/fold

**Full name:** `taxi.stdlib.fold`

## Signature

```taxi
declare extension function <T,A> fold(collection: T[], initial: A, callback: (T,A) -> A):A
```

## Description

Iterates over a collection, combining elements with an accumulator to produce a single result.

Unlike `reduce`, `fold` starts with an explicit `initial` value as the accumulator.

```taxi
// Compute a weighted sum
model Entry {
   weight: Weight inherits Int
   score: Score inherits Int
}
type WeightedTotal by (Entry[]) -> Entry[].fold(0, (Entry, Int) -> Int + (Weight * Score))
```