## MLton 20241230

# FunctionalRecordUpdate

Functional record update is the copying of a record while replacing the values of some of the fields. Standard ML does not have explicit syntax for functional record update. We will show below how to implement functional record update in SML, with a little boilerplate code.

As an example, the functional update of the record

    {a = 13, b = 14, c = 15}

with `c = 16` yields a new record

    {a = 13, b = 14, c = 16}

Functional record update also makes sense with multiple simultaneous updates. For example, the functional update of the record above with `a = 18, c = 19` yields a new record

    {a = 18, b = 14, c = 19}

One could easily imagine an extension of the SML that supports functional record update. For example

    e with {a = 16, b = 17}

would create a copy of the record denoted by `e` with field `a` replaced with `16` and `b` replaced with `17`.

Since there is no such syntax in SML, we now show how to implement functional record update directly. We first give a simple implementation that has a number of problems. We then give an advanced implementation, that, while complex underneath, is a reusable library that admits simple use.

## Simple implementation

To support functional record update on the record type

    {a: 'a, b: 'b, c: 'c}

first, define an update function for each component.

    fun withA ({a = _, b, c}, a) = {a = a, b = b, c = c}
    fun withB ({a, b = _, c}, b) = {a = a, b = b, c = c}
    fun withC ({a, b, c = _}, c) = {a = a, b = b, c = c}

Then, one can express `e with {a = 16, b = 17}` as

    withB (withA (e, 16), 17)

With infix notation

    infix withA withB withC

the syntax is almost as concise as a language extension.

    e withA 16 withB 17

This approach suffers from the fact that the amount of boilerplate code is quadratic in the number of record fields. Furthermore, changing, adding, or deleting a field requires time proportional to the number of fields (because each `with<L>` function must be changed). It is also annoying to have to define a `with<L>` function, possibly with a fixity declaration, for each field.

Fortunately, there is a solution to these problems.

## Advanced implementation

Using Fold one can define a family of `makeUpdate<N>` functions and single update operator `U` so that one can define a functional record update function for any record type simply by specifying a (trivial) isomorphism between that type and function argument list. For example, suppose that we would like to do functional record update on records with fields `a` and `b`. Then one defines a function `updateAB` as follows.

    val updateAB =
       fn z =>
       let
          fun from v1 v2 = {a = v1, b = v2}
          fun to f {a = v1, b = v2} = f v1 v2
       in
          makeUpdate2 (from, from, to)
       end
       z

The functions `from` (think from function arguments) and `to` (think to function arguements) specify an isomorphism between `a`,`b` records and function arguments. There is a second use of `from` to work around the lack of first-class polymorphism in SML.

With the definition of `updateAB` in place, the following expressions are valid.

    updateAB {a = 13, b = "hello"} (set#b "goodbye") $
    updateAB {a = 13.5, b = true} (set#b false) (set#a 12.5) $

As another example, suppose that we would like to do functional record update on records with fields `b`, `c`, and `d`. Then one defines a function `updateBCD` as follows.

    val updateBCD =
       fn z =>
       let
          fun from v1 v2 v3 = {b = v1, c = v2, d = v3}
          fun to f {b = v1, c = v2, d = v3} = f v1 v2 v3
       in
          makeUpdate3 (from, from, to)
       end
       z

With the definition of `updateBCD` in place, the following expression is valid.

    updateBCD {b = 1, c = 2, d = 3} (set#c 4) (set#c 5) $

Note that not all fields need be updated and that the same field may be updated multiple times. Further note that the same `set` operator is used for all update functions (in the above, for both `updateAB` and `updateBCD`).

In general, to define a functional-record-update function on records with fields `f1`, `f2`, …​, `fN`, use the following template.

    val update =
       fn z =>
       let
          fun from v1 v2 ... vn = {f1 = v1, f2 = v2, ..., fn = vn}
          fun to f {f1 = v1, f2 = v2, ..., fn = vn} = v1 v2 ... vn
       in
          makeUpdateN (from, from, to)
       end
       z

With this, one can update a record as follows.

    update {f1 = v1, ..., fn = vn} (set#fi1 vi1) ... (set#fim vim) $

## The `FunctionalRecordUpdate` structure

Here is the implementation of functional record update.

    structure FunctionalRecordUpdate =
       struct
          local
             fun next g (f, z) x = g (f x, z)
             fun f1 (f, z) x = f (z x)
             fun f2  z = next f1  z
             fun f3  z = next f2  z

             fun c0  from = from
             fun c1  from = c0  from f1
             fun c2  from = c1  from f2
             fun c3  from = c2  from f3

             fun makeUpdate cX (from, from', to) record =
                let
                   fun ops () = cX from'
                   fun vars f = to f record
                in
                   Fold.fold ((vars, ops), fn (vars, _) => vars from)
                end
          in
             fun makeUpdate0  z = makeUpdate c0  z
             fun makeUpdate1  z = makeUpdate c1  z
             fun makeUpdate2  z = makeUpdate c2  z
             fun makeUpdate3  z = makeUpdate c3  z

             fun upd z = Fold.step2 (fn (s, f, (vars, ops)) => (fn out => vars (s (ops ()) (out, f)), ops)) z
             fun set z = Fold.step2 (fn (s, v, (vars, ops)) => (fn out => vars (s (ops ()) (out, fn _ => v)), ops)) z
          end
       end

The idea of `makeUpdate` is to build a record of functions which can replace the contents of one argument out of a list of arguments. The functions `f<X>` replace the 0th, 1st, …​ argument with their argument `z`. The `c<X>` functions pass the first X `f` functions to the record constructor.

The `#field` notation of Standard ML allows us to select the map function which replaces the corresponding argument. By converting the record to an argument list, feeding that list through the selected map function and piping the list into the record constructor, functional record update is achieved.

## Efficiency

With MLton, the efficiency of this approach is as good as one would expect with the special syntax. Namely a sequence of updates will be optimized into a single record construction that copies the unchanged fields and fills in the changed fields with their new values.

Before Sep 14, 2009, this page advocated an alternative implementation of FunctionalRecordUpdate. However, the old structure caused exponentially increasing compile times. We advise you to switch to the newer version.

## Applications

Functional record update can be used to implement labelled optional arguments.