Expression sub-language for modifying state

[jeromesimeon]

When changing the state, one has currently to rebuild the whole state each time. It might be useful to have expressions specifically designed to modify the state. For instance:

set status to "BREACHED" in state;
add new PaymentObligation to obligations in state;
remove notice in obligations from state;
return ...

[mttrbrts]

How about something like ...

set state.status to "BREACHED";
add new PaymentObligation to state.obligations;
remove notice from state.obligations;
return

[jeromesimeon]

I like it! One issue is what does remove mean, for which there are several options:

• is remove based on value equality (i.e., notice is an obligation object and your compare the structure)
• is remove based on a kind of identity (we never use identifiers anywhere at the moment)
• is remove based on another kind of criteria

We could imagine something like the following, which would provide a lot more flexibility as to how the user specifies what to remove:

remove x from state.obligations where x.kind = "notice";
remove x from state.obligations where x.id = "5555-3562-1412";
return ...

[jeromesimeon]

Another question, is whether those new operations should replace the current set state statement altogether (i.e., remove the current set state).

[dselman]

For types that have identified by in the model (basically everything except concepts and enums) I think it would be natural to use the modelled identifier for equality. For concepts I guess we either have to use value-based equality, or allow the user to specify an identified by expression to the call to remove.

I do like the where expressions for selection as well however.

How about update state to contrast with set state?

[jeromesimeon]

@dselman I am not sure if we have a mechanism to check the unicity of identifiers at the moment? And they aren't distinguished from other attributes in Ergo (yet?) -- technically there isn't any object equality in Ergo, only value equality.

You could easily use the where form to handle it as well:

remove x from state.obligations where x.id = notice_id;

If we keep both forms, I think the update state and set state distinction may help, but isn't required.

[jeromesimeon]

For the records, @kach had suggested the nice and compact:

state.status <- BREACHED;

And add/remove can technically be handled using array operations:

state.obligations <- arrayAdd(state.obligations, [new PaymentObligation{ ... }]);
state.obligations <- arraySubtract(state.obligations, notice);

[mttrbrts]

I like the brevity of this suggestion, but would rather avoid introducing another operator.

With this in mind what about ... ?

set state.status = BREACHED;

This aligns the syntax for state assignment with assignment in let expressions.

[otawakkil]

There is a nice syntax in F# (OCaml Dotnet variant) that does something like this
set state { state with status = "STARTED" }
It is aligned with the "immutability" mindset.
the expression is like
{ objReference 'with' fieldAssignmentList }

[jeromesimeon]

I like the idea of conveying cloning since this could be used outside of setting state. I don’t know why I didn’t think of this. And of course the ocaml/f# syntax is quite nice for that.

Note that things are a bit more difficult in Ergo since we need those operations to be aware of the “class hierarchy”, not just records as in OCaml.

A branch is tracking this, but I still need to work through the typing ramifications.

[otawakkil]

Is there a level of meta-programming or reflection on types ?

[jeromesimeon]

Not in Ergo itself if I understand your question correctly. Although you can use match expression on type names which are declared as part of the hierarchy.

[jeromesimeon]

In JavaScript, we can use the spread operator for cloning. This might be a good syntax in Ergo as well?

set state { ...state, "status": "BREACHED" }

[otawakkil]

Nice. You can even save typing by using & instead

[dselman]

In JavaScript, we can use the spread operator for cloning. This might be a good syntax in Ergo as well?

set state { ...state, "status": "BREACHED" }

Doesn't seem that obvious to a less technical user. update state ?

[jeromesimeon]

@dselman that was also the impression on the Ergo call this morning.

I do find a syntax for cloning appealing since it can be applied to things other than the state, but it's maybe something to wait on.

[otawakkil]

The logic is to be able to clone and override any type not just the state.
let myObj = {...someBojOfSameType, overridenFieldList }

state being here just one case of it.

[jeromesimeon]

I like the brevity of this suggestion, but would rather avoid introducing another operator.

With this in mind what about ... ?

set state.status = BREACHED;

This aligns the syntax for state assignment with assignment in let expressions.

This is the proposal that has been implemented in #667

Note that this new feature is specific to the state, this does not include support for things like spread operator.

[jeromesimeon]

Implemented in the 0.20 branch. Tested in Cicero and Cicero template library.