# The Worker/Wrapper Transformation

The **worker/wrapper transformation** is a technique for transforming a
computation of one type into a *worker* of a different type, together
with a *wrapper* that acts as an impedance matcher between the original
and new computations. The technique can be used to improve the
performance of functional programs by improving the choice of data
structures used.

## Transformation Overview

(click to enlarge)

This image is an abstract diagram explaining the worker/wrapper
transformation, where arrows signify usage. The (smaller) boxes are
computations that have a type, given as an incoming arrow on the left
hand side, and require the use of a second computation, given as an
outgoing arrow on the right hand side. These computational boxes can be
considered representing functions with higher-order arguments. **A** and
**B** should be read as providing a typed service by using the box to
their right to provide this service.

The **original computation** is a recursive function, that is it uses
itself, notated with a right arrow that arcs back to the type of the
computation, **A**. The ultimate result is a wrapper and an optimized
worker, which operates using a new type, **B**.

The worker/wrapper transformation operates as follows:

- A target type for the recursion is selected, called
**B**. - A
**coercion chain**is constructed from two higher-order functions that coerce from**A**to**B**, and back to**A**. - The worker/wrapper local preconditions are verified; one example is
when
**abs**and**rep**form an identity. - The worker/wrapper transformation is applied, replacing then the
original computation with the
**post worker/wrapper chain**. - This new formulation can be transformed and optimized using well
understood local refinements into a
*worker*that operates efficiently over**B**, and a*wrapper***abs**that allows original users of**A**to call this new function.

This general pattern captures many possible transformations and refinements.

## Worker/Wrapper Theorem

The transformation can be expressed formally, using the theory of
**least fixed points** over pointed ω-complete partial orders (the
semantic domain of Haskell).

### Worker/Wrapper Factorization Rule

If **comp :: A** is defined by **comp = fix body** for some **body ::
A→A**, and **rep :: A→B** and **abs :: B→A** satisfy any of the
worker/wrapper assumptions, then

**comp = abs work**

where **work :: B** is defined by

**work = fix (rep ο body ο abs)**

Alternatively, **work** can be defined as

**work = rep comp**

which avoids the need to define **comp** using an explicit fixed point.

### Worker/Wrapper Assumptions

There are several worker/wrapper assumptions that are sufficient to justify worker/wrapper factorization. The three given in the worker/wrapper paper are:

```
| Rule | Rule Name --------|----------------------------------------|----------------------------- **(A)** | **abs ο rep = id** | (basic assumption) **(B)** | **abs ο rep ο body = body** |(body assumption) **(C)** | **fix (abs ο rep ο body) = fix body** |(fixed-point assumption)
```

There is also another possible assumption, which uses the **fixed point
fusion** rule (hence the name fpf assumption) and only holds for a
strict **abs**:

**(D)** **wrap ο rep ο body ο abs = body ο abs** (fpf assumption)
——- —————————————- —————–

### Worker/Wrapper Fusion Rule

If any of the worker/wrapper assumptions **(A)**, **(B)** or **(C)**
hold, then:

**rep (abs work) = work**

## Papers

N. Sculthorpe and G. Hutton, “Work it, wrap it, fix it, fold it,”

*Journal of Functional Programming*, vol. 24, no. 1, pp. 113–127, 2014.

A. Gill and G. Hutton, “The worker/wrapper transformation,”

*Journal of Functional Programming*, vol. 19, pp. 227–251, March 2009.

## Use Cases

The Worker/Wrapper Transformation is surprisingly general! It has been applied to the following application areas:

**Using strictness information**—was the original motivating example for this transformation.**CPS translation**—is just using an alternative representation for computation.**Memoization**—is using a data-structure to represent a function.**Accumulation**—is often possible when the result is a monoid; worker/wrapper enables this change of type.**Constructor specialization**—creates specialized workers and wrappers.**Cross-function short-cut fusion**—uses worker/wrapper to transmit fusion opportunities over function boundaries.

We expect there are many others.