Copy on write semantics for collections

[anderson-joyle]

Introduced by PR #1512.

Set with value semantics

Collections are just global variables that support mutation. When changing a collection through a mutation function (Set, Collect, Patch, Remove, etc), Power Fx creates copies (shallow copy always) of in-memory global variables and collections

Examples

>> Set(x, [1,2,3]
[1,2,3]

>> Set(y, x) // copy of 'x'
[1,2,3]

>> Collect(x, {Value:4})
{Value:4}

>> Remove(x, {Value:2})
true

>> x // Changed. Expected.
[1,3,4]

>> y // Unchanged. Expected.
[1,2,3]

>> Patch(y, First(y), {Value:99})
{Value:99}

>> First(y)
{Value:99}

>> First(x)
{Value:1}

The mutation functions return values already merged.

Deep mutation with Set

Set can modify fields and records deep within a data structure. In the following examples, it will produce a shallow copy of orig but reference is retained to individual records.

Examples

>> Set( orig, [ {a:1, b:2}, {a:3, b:4} ] )
Table({a:1, b:2}, {a:3, b:4})

>> Set( x, orig )
Table({a:1, b:2}, {a:3, b:4})

>> Set( y, orig[1] )
{a:3, b:4}

>> Set( orig[1].b, 9 )
9

>> Notify( x[1].b )
2

>> Notify( y.b )
2