### coq

#### How do you selectively simplify arguments to each time a function is called, without evaluting the function itself?

```I'm using Coq 8.5pl1.
To make a contrived but illustrative example,
(* fix so simpl will automatically unfold. *)
Definition double := fix f n := 2*n.
Theorem contrived n : double (2 + n) = 2 + double (1 + n).
Now, I only want to simplify the arguments to double,
and not any part outside of it. (For example, because the
rest has already carefully been put into the correct form.)
simpl.
S (S (n + S (S (n + 0)))) = S (S (S (n + S (n + 0))))
This converted the outside (2 + ...) to (S (S ...)) as well as unfolding double.
I can match one of them by doing:
match goal with | |- (double ?A) = _ => simpl A end.
double (S (S n)) = 2 + double (1 + n)
How do I say that I want to simplify all of them?
That is, I want to get
double (S (S n)) = 2 + double (S n)
without having to put a separate pattern for each call to double.
I can simplify except for double itself with
remember double as x; simpl; subst x.
double (S (S n)) = S (S (double (S n)))
```
```Opaque/Transparent approach
Combining the results of this answer and this one, we get the following solution:
Opaque double.
simpl (double _).
Transparent double.
We use the pattern capability of simpl to narrow down its "action domain", and Opaque/Transparent to forbid (allow resp.) unfolding of double.
Custom tactic approach
We can also define a custom tactic for simplifying arguments:
(* simplifies the first argument of a function *)
Ltac simpl_arg_of function :=
repeat multimatch goal with
| |- context [function ?A] =>
let A' := eval cbn -[function] in A in
change A with A'
end.
That let A' := ... construction is needed to protect nested functions from simplification. Here is a simple test:
Fact test n :
82 + double (2 + n)
=
double (1 + double (1 + 20)) + double (1 * n).
Proof.
simpl_arg_of double.
The above results in
82 + double (S (S n)) = double (S (double 21)) + double (n + 0)
Had we used something like context [function ?A] => simpl A in the definition of simpl_arg_of, we would've gotten
82 + double (S (S n)) = double 43 + double (n + 0)
Arguments directive approach
As suggested by #eponier in comments, we can take advantage of yet another form of simpl -- simpl <qualid>, which the manual (sect. 8.7.4) defines as:
This applies simpl only to the applicative subterms whose head occurrence is the unfoldable constant qualid (the constant can be referred to by its notation using string if such a notation exists).
The Opaque/Transparent approach doesn't work with it, however we can block unfolding of double using the Arguments directive:
Arguments double : simpl never.
simpl double.
```
```You may find the ssreflect pattern selection language and rewrite mechanism useful here. For instance, you can have a finer grained control with patterns + the simpl operator /=:
From mathcomp Require Import ssreflect.
Definition double := fix f n := 2*n.
Theorem contrived n : double (2 + n) = 2 + double (1 + n).
rewrite ![_+n]/=.
Will display:
n : nat
============================
double (S (S n)) = 2 + double (S n)
You can also use anonymous rewrite rules:
rewrite (_ : double (2+n) = 2 + double (1+n)) //.
I would personally factor the rewrite in smaller lemmas:
Lemma doubleE n : double n = n + n.
Proof. by elim: n => //= n ihn; rewrite -!plus_n_Sm -plus_n_O. Qed.
Lemma doubleS n : double (1 + n) = 2 + double n.
Proof. by rewrite !doubleE /= -plus_n_Sm. Qed.
Theorem contrived n : double (1+n) = 2 + double n.
rewrite doubleS.```

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