In a nutshell, this library automatically provides a generic way to update record fields. Here's a teaser example:

From RecordUpdate Require Import RecordSet.

Record X := mkX { A: nat; B: nat; C: bool; }.

(* all you need to do is provide something like this, listing out the fields of your record: *)
#[export] Instance etaX : Settable _ := settable! mkX <A; B; C>.

(* and now you can set fields! *)
Definition setAB a b x := set B (fun _ => b) (set A (fun _ => a) x).

(* and do updates that use the old value! *)
Definition updateAB a b x := set B (Nat.add b) (set A (Nat.add a) x).

(* you can also use notations for these things: *)
Import RecordSetNotations.
Definition setAB' a b x := x <|A := a|> <|B := b|>.
Definition updateAB' a b x := x <|A ::= Nat.add a|> <|B ::= Nat.add b|>.

(* the notation also allows you to update nested fields: *)
Record C := mkC { n : nat }.
Record B := mkB { c : C }.
Record A := mkA { b : B }.

Instance etaC : Settable _ := settable! mkC<n>.
Instance etaB : Settable _ := settable! mkB<c>.
Instance etaA : Settable _ := settable! mkA<b>.

Definition setNested n' x := x <| b; c; n := n' |>.
Definition incNested x := x <| b; c; n ::= S |>.

Coq has no record update syntax, nor does it create updaters for setting individual fields of a record. This small library automates creating such updaters.

To use the library with a record, one must implement a typeclass Settable to provide the syntax for constructing a record from individual fields. This implementation lists out the record's constructor and every field accessor function. If you want to get rid of that boilerplate, you can, but it requires an OCaml plugin so it is provided separately; see tchajed/coq-record-update-plugin.

Once Settable T is implemented, Coq will be able to resolve the typeclass Setter F for all the fields F of T, so that a generic setter set T A (F: T -> A) : forall {_:Setter F}, A -> T -> T works. There is also a notation x <| proj := v |> for calling set proj v x.

As a bonus, the Setter F typeclass includes some theorems showing the updater is correct. In addition, Settable T has a theorem showing that the fields are listed correctly. Together, these ensure that the library cannot be used incorrectly; for Setter this catches potential bugs in the library, while the property in Settable ensures that fields aren't listed out-of-order or duplicated.

If you have feedback or need some improvement to make this library useful to you, please open an issue. I do actively maintain it, though that has only required the occasional bug fix for a while.

To build and install:

git clone https://github.com/tchajed/coq-record-update.git
cd coq-record-update
make   # or make -j <number-of-cores-on-your-machine>
make install

I'm glad you asked! There are three tricks here:

1. First, we represent the fields of the record. The representation is actually just an identity function for the record, but it re-constructs the record from its fields; for example, it might look like fun x => mkX (A x) (B x) (C x). I think of this expression as the record's eta expansion, since it deconstructs the record and then re-assembles it.

2. The second trick is that we can take this identity function and make a small tweak to it to turn it into an updater for a single field: if we replace a field with f: R -> T in the eta expansion (where R is the record type and T is the field type), instead of putting the field back as-is, we can substitute some update function.

   To actually implement this substitution without doing it by hand, we use the pattern tactic. This is easiest to illustrate with an example: pattern field2 in (fun x => mkX (field1 x) (field2 x) (field3 x)) evaluates to (fun f => (fun x => mkX (field1 x) (f x) (field3 x))) field2. The first function is essentially the updater we want! We can now extract it with a simple Ltac pattern match. We do make one tweak which is rather than allowing the user to pass any function of the whole record, of type R -> T, we only allow a function of the current field value, of type T -> T.

3. The final piece of the puzzle is to get all of this Ltac to run. Here we (abuse) typeclasses, in two ways. You might notice that the set function in coq-record-update is just part of the class Setter r field. To resolve that class, we use a tactic rather than user-provided instances, and that tactic implements the pattern trick --- the tactic is easy to install because typeclass resolution is just an auto-like search using the typeclass_instances hint database, and we can sneak a Hint Extern into that database. That's the first typeclass trick. The second is used to look up the record eta expansion when resolving Setter r field. Here we have the user write a typeclass Settable r with the eta expansion and in Ltac we look up the eta expansion and then unfold it to look at the syntax, since the actual expression is relevant and not just its use as a function. In fact, you can implement Settable by providing the identity function and then setting won't work because the Ltac can't do anything with it.

It's pretty cool what you can do with Coq typeclasses.