SML record subset and update

Question

Is there an more straight forward way (and less error prone due to maintenance and amount of editing involved), to achieve the same as below, that is: 1) getting a subset of the elements of a record, given the names list of members of interest, 2) and updating a record from a subset record, using the names of the subset record?

val r = {a = 1, b = 2, c = 3, d = 4, e = 5}
val s = {b = #b r, c = #c r, d = #d r}
val r = {a = #a r, b = #b s, c = #c s, d = #d s, e = #e r}

What I expect, could look like this:

val r = {a = 1, b = 2, c = 3, d = 4, e = 5}
val s = #(b, c, d) r
val r = r / s

As Sebastian Paaske Tørhol said, one can extract part of a record this way:

val {b, c, d, ...} = r

Unfortunately, this does not bind the subset to a single name, as this, as an example, does not work:

val s = {b, c, d, ...} = r

Answer 1

No, there isn't. There once was a proposal for more extensive record operations (Functional record extension and row capture), as part of the short-lived successor ML effort. With that, you could have written, e.g.,

val r = {a = 1, b = 2, c = 3, d = 4, e = 5}
val {a, e, ... = s} = r
val t = {a = a+1, e = e-1, ... = s}

Unfortunately, nobody ever implemented it in a real compiler.

Answer 2

You can use pattern-matching to extract relevant parts of records:

Example:

val question = { title = "SML record subset and update",
                 author = "Hibou57",
                 tags = ["SML"] }

val { title = t, tags = ts, ... } = question

This binds title into t and tags into ts.

You can also simply omit the names, if you so desire:

val { title, tags, ... } = question

This binds the title to title and tags to tags.

As far as updating goes, Standard ML sadly doesn't have a nice way to update records without explicitly having to set every field.