Invocation of Elpi code

Elpi <qname> <argument>*. invokes the main predicate of the <qname> program passing a possible empty list of arguments. This is how you invoke a command.
elpi <qname> <argument>*. invokes the solve predicate of the <qname> program passing a possible empty list of arguments and the current goal. This is how you invoke a tactic.
Elpi Export <qname> [As <other-qname>] makes it possible to invoke command <qname> (or <other-qname> if given) without the Elpi prefix, or invoke tactic <qname> in the middle of a term just writing <qname> args instead of ltac:(elpi <qname> args). Note that in the case of tactics, all arguments are considered to be terms. Moreover, remember that one can use Tactic Notation to give the tactic a better syntax and a shorter name when used in the middle of a proof script. Commands can declare the behavior of starting/ending a proof by, respectively, using #[proof="begin"] Elpi Export .. and #[proof="end"] Elpi Export ... Starting the proof can depend on the presence of an attribute, for example #[proof(begin_if="interactive")] Elpi Export foo will make foo not open a proof, while #[interactive] foo will open a proof. See also the main-interp-proof and main-interp-qed entry points in Builtins and relevant files.

where <argument> can be:

a number, e.g. 3, represented in Elpi as (int 3)
a string, e.g. "foo" or bar.baz, represented in Elpi as (str "foo") and (str "bar.baz"). Rocq keywords and symbols are recognized as strings, e.g. => requires no quotes. Quotes are necessary if the string contains a space or a character that is not accepted for qualified identifiers, or if the string is Definition, Axiom, Record, Structure, Inductive, CoInductive, Variant or Context.
a term, e.g. (3) or (f x), represented in Elpi as (trm ...). Note that terms always require parentheses, that is 3 is a number while (3) is a Rocq term and depending on the context could be a natural number (i.e. S (S (S O))) or a Z or ... See also the sections below on terms as arguments and Ltac variables.

Commands also accept the following arguments (the syntax is as close as possible to the Rocq one: [...] means optional, * means 0 or more). See the argument data type in Builtins and relevant files for their HOAS encoding. See also the section on terms as arguments below.

Definition name binder* [: term] := term
Axiom name : term
[ Record | Structure ] name binder* [: sort] := [name] { name : term ; * }
[ Inductive | CoInductive | Variant ] name binder* [| binder*] [: term] := | name binder* : term *
Context binder*

Ltac variables

Tactics also accept Ltac variables as follows:

ltac_string:(v) (for v of type string or ident)
ltac_int:(v) (for v of type int or integer)
ltac_term:(v) (for v of type constr or open_constr or uconstr or hyp)
ltac_open_term:(v) (for v of type uconstr)
ltac_(string|int|term|open_term)_list:(v) (for v of type list of ...)
ltac_tactic:(t) (for t of type tactic)
ltac_attributes:(v) (for v of type attributes)

For example:

Tactic Notation "tac" string(X) ident(Y) int(Z) hyp(T) constr_list(L) simple_intropattern_list(P) uconstr(U) tactic(TA) :=
  elpi tac ltac_string:(X) ltac_string:(Y) ltac_int:(Z) ltac_term:(T) ltac_term_list:(L) ltac_tactic:(intros P) ltac_open_term:(U) ltac_tactic:(TA).

lets one write tac "a" b 3 H t1 t2 t3 [|m] u ta in any Ltac context. Arguments are first interpreted by Ltac according to the types declared in the tactic notation and then injected in the corresponding Elpi argument. For example H must be an existing hypothesis, since it is typed with the hyp Ltac type, but in Elpi it will appear as a term, e.g. trm c0. Similarly t1, t2 and t3 are checked to be well typed and to contain no unresolved implicit arguments, since this is what the constr Ltac type means. If they were typed as open_constr or uconstr, the last or both checks would be respectively skipped. In any case they are passed to the Elpi code as trm .... Both "a" and b are passed to Elpi as str .... Argument U flagged as ltac_open_term can mention free variables. The Elpi tactic receives open-trm N F where N is the number of free variables in U and F is fun x1 => ... fun xN => U. Argument TA is received as tac T where T is an (opaque) tactic that can be called via the coq.ltac.* APIs. Finally, ltac_term:(T) and (T) are not synonyms: the former must be used when defining tactic notations, the latter when invoking elpi tactics directly. \`(T) can be used to pass an open term to elpi tactic ....

Attributes

Attributes are supported in both commands and tactics. Examples:

#[ att ] Elpi cmd
#[ att ] cmd for a command cmd exported via Elpi Export cmd
#[ att ] elpi tac
Tactic Notation ... attributes(A) ... := ltac_attributes:(A) elpi tac. Due to a parsing conflict in Rocq's grammar, at the time of writing this code:
```
Tactic Notation "#[" attributes(A) "]" "tac" :=
  ltac_attributes:(A) elpi tac.
```
has the following limitation:
- #[ att ] tac. does not parse
- (#[ att ] tac). works
- idtac; #[ att ] tac. works

Terms as arguments

Since version 1.15, terms passed to Elpi commands via (term) or via a declaration (like Record, Inductive ...) are in elaborated format by default. This means that all Rocq notational facilities are available, like deep pattern matching, or tactics in terms. One can use the attribute #[arguments(raw)] to declare a command which instead takes arguments in raw format. In that case, notations are unfolded, implicit arguments are expanded (holes _ are added) and lexical analysis is performed (global names and bound names are identified, holes are applied to bound names in scope), but deep pattern matching or tactics in terms are not supported, and in particular type checking/inference is not performed. One can use the coq.typecheck or coq.elaborate-skeleton APIs to fill in implicit arguments and insert coercions on raw terms.

Terms passed to Elpi tactics via tactic notations can be forced to be elaborated beforehand by declaring the parameters to be of type constr or open_constr. Arguments of type uconstr are passed raw.

Testing/debugging

Elpi Query [<qname>] <code> runs <code> in the current program (or in <qname> if specified).
Elpi Query [<qname>] <synterp-code> <interp-code> runs <synterp-code> in the current (synterp) program (or in <qname> if specified) and <interp-code> in the current program (or <qname>).
elpi query [<qname>] <string> <argument>* runs the <string> predicate (that must have the same signature as the default predicate solve).