Question

Does the compiler create a new location in memory when a record is extended (deep copy?) or does the compiler make the record mutable and modify the value?

For example:

type MyRecord = { A : string
                ; B : string
                }

let record = { A = "A"; B = "B" }

let record = { record with A = "new A" } //copy or overwrite? 

Since I am overwriting record does the compiler copy or overwrite? Are there performance concerns either way?

Answer 1

It makes the copy.

Copy-and-update Record expression

*A copy-and-update record expression elaborates as if it were a record expression written as follows:

let v = expr in { field-label1 = expr1 ; … ; field-labeln = exprn; F1 = v.F1; … ; FM = v.FM }

where F1 … FM are the fields of R that are not defined in field-initializers and v is a fresh variable.*