[MLton] A variation of functional record update notation

Vesa Karvonen vesa.karvonen@cs.helsinki.fi
Mon, 8 Aug 2005 06:07:09 +0300

```The functional record update technique (http://mlton.org/FunctionalRecordUpdate)
uses an infix update operator, e.g. <<, to make chained updates readable:

record << (#field1, value1)
<< (#field2, value2)
...
<< (#fieldN, valueN)

It occured to me that if you curry the update function and reorder the
parameters, you can use the left piping operator (http://mlton.org/InfixingOperators)
and still have a fairly readable notation:

record >| set#field1 value1
>| set#field2 value2
...
>| set#fieldN valueN

Above, `set' refers to the update function. The syntax is fairly readable
even when the update function is referred to by a longvid:

record >| R.set#field1 value1
>| R.set#field2 value2
...
>| R.set#fieldN valueN

This way there is no need to define a separate infix operator for functional
record update.

The `fru3' function is changed to:

fun makeSetter3 (r1, r2, t) f v r =
let
datatype ('a, 'b, 'c) t = A of 'a | B of 'b | C of 'c
val (a, b, c) = t r
fun g h value =
r1 (case h value of A a => a | _ => a,
case h value of B b => b | _ => b,
case h value of C c => c | _ => c)
in
f (r2 (g A, g B, g C)) v
end

A `set' function can be made like this:

fun set f v s =
let
fun r (x1, x2, x3) = {a = x1, b = x2, c = x3}
fun t {a = x1, b = x2, c = x3} = (x1, x2, x3)
in
makeSetter3 (r, r, t) f v s
end

The `set' function can be used like this:

{a = 1, b = 2, c = 3} >| set#a ~1
>| set#b ~2

-Vesa Karvonen

```