This seems like a really simple question, but I wasn't able to find anything useful.
I have the statement
n - x = n
and would like to prove
(n - x) + x = n + x
I haven't been able to find what theorem allows for this.
You should have a look at the rewrite tactic (and then maybe reflexivity).
EDIT: more info about rewrite:
You can rewrite H rewrite -> H to rewrite from left to right
You can rewrite <- H to rewrite from right to left
You can use the pattern tactic to only select specific instances of the goal to rewrite. For example, to only rewrite the second n, you can perform the following steps
pattern n at 2.
rewrite <- H.
In your case, the solution is much simpler.
Building on #gallais' suggestion on using f_equal. We start in the following state:
n : nat
x : nat
H : n - x = n
============================
n - x + x = n + x
(1) First variant via "forward" reasoning (where one applies theorems to hypotheses) using the f_equal lemma.
Check f_equal.
f_equal
: forall (A B : Type) (f : A -> B) (x y : A), x = y -> f x = f y
It needs the function f, so
apply f_equal with (f := fun t => t + x) in H.
This will give you:
H : n - x + x = n + x
This can be solved via apply H. or exact H. or assumption. or auto. ... or some other way which suits you the most.
(2) Or you can use "backward" reasoning (where one applies theorems to the goal).
There is also the f_equal2 lemma:
Check f_equal2.
f_equal2
: forall (A1 A2 B : Type) (f : A1 -> A2 -> B)
(x1 y1 : A1) (x2 y2 : A2),
x1 = y1 -> x2 = y2 -> f x1 x2 = f y1 y2
We just apply it to the goal, which results in two trivial subgoals.
apply f_equal2. assumption. reflexivity.
or just
apply f_equal2; trivial.
(3) There is also the more specialized lemma f_equal2_plus:
Check f_equal2_plus.
(*
f_equal2_plus
: forall x1 y1 x2 y2 : nat,
x1 = y1 -> x2 = y2 -> x1 + x2 = y1 + y2
*)
Using this lemma we are able to solve the goal with the following one-liner:
apply (f_equal2_plus _ _ _ _ H eq_refl).
There is a powerful search engine in Coq using patterns. You can try for example:
Search (_=_ -> _+_=_+_).
Related
I have two hypothesis, and I would like to use forward reasoning to apply a theorem that uses both of them.
My specific I have the hypothes
H0 : a + b = c + d
H1 : e + f = g + h
and I want to apply the theorem from the standard library:
f_equal2_mult
: forall x1 y1 x2 y2 : nat, x1 = y1 -> x2 = y2 -> x1 * x2 = y1 * y2
Now I know I could manually give the values for x1, y1, x2, y2, but I would like Coq to automatically determine those values when it unified with H0 and H1. I have figured out that I can get it to work like so:
eapply f_equal2_mult in H0; try exact H1.
But this feels like a hack, with the broken symmetry and the try. I really would like to be able to say apply f_equals2_mult in H0, H1 or something similarly clear. Is there a way like this?
You could use pose proof to introduce the lemma in the context, and specialize to apply it to other hypotheses.
Lemma f (a b c d : nat) : a = b -> c = d -> False.
intros H1 H2.
pose proof f_equal2_mult as pp.
specialize pp with (1 := H1).
specialize pp with (1 := H2).
(* or *)
specialize pp with (1 := H1) (2 := H2).
With an inductive definition like:
Inductive A :=
mkA : nat -> A.
Proving constructors are partial functions can be encoded as:
Lemma constructor_functional :
forall i1 i2, mkA i1 <> mkA i2 -> i1 <> i2.
Although it is trivial to prove, doing so for every defined type sounds weird.
Is there a tactic to encode this property? Or some equivalent in a library? I did not find anything in ssreflect though by searching (_ <> _).
You can state a generic lemma that subsumes this result for every Coq function. Since constructors like your mkA are just functions, the result applies to them as well.
Lemma function_functional :
forall (X Y : Type) (f : X -> Y) (x1 x2 : X),
f x1 <> f x2 -> x1 <> x2.
Proof.
intros X Y f x1 x2 H1 H2.
apply H1.
now rewrite H2.
Qed.
This statement is actually the contrapositive of the following one from the standard library.
Lemma f_equal :
forall (X Y : Type) (f : X -> Y) (x1 x2 : X),
x1 = x2 -> f x1 = f x2.
I'm new to Coq and am doing some exercises to get more familiar with it.
My understanding is that proving a proposition in Coq "really" is writing down a type in Gallina and then showing that it's inhabited using tactics to combine terms together in deterministic ways.
I'm wondering if there's a way to get a pretty-printed representation of the actual term, with all the tactics removed.
In the example below, an anonymous term of type plus_comm (x y : N) : plus x y = plus y x is ultimately produced... I think. What should I do if I want to look at it? In a certain sense, I'm curious what the tactics "desugar" to.
Here's the code in question, lifted essentially verbatim from a tutorial on YouTube https://www.youtube.com/watch?v=OaIn7g8BAIc.
Inductive N : Type :=
| O : N
| S : N -> N
.
Fixpoint plus (x y : N) : N :=
match x with
| O => y
| S x' => S (plus x' y)
end.
Lemma plus_0 (x : N) : plus x O = x.
Proof.
induction x.
- simpl. reflexivity.
- simpl. rewrite IHx. reflexivity.
Qed.
Lemma plus_S (x y : N) : plus x (S y) = S(plus x y).
Proof.
induction x.
- simpl. reflexivity.
- simpl. rewrite IHx. reflexivity.
Qed.
Lemma plus_comm (x y : N) : plus x y = plus y x.
Proof.
induction x.
- simpl. rewrite plus_0. reflexivity.
- simpl. rewrite IHx. rewrite plus_S. reflexivity.
Qed.
First of all, plus_comm is not a part of the type. You get a term named plus_comm of type forall x y : N, plus x y = plus y x. You can check it using the following command
Check plus_comm.
So, an alternative way of defining the plus_comm lemma is
Lemma plus_comm : forall x y : N, plus x y = plus y x.
As a side note: in this case you'll need to add intros x y. (or just intros.) after the Proof. part.
Tactics (and the means to glue them together) are a metalanguage called Ltac, because they are used to produce terms of another language, called Gallina, which is the specification language of Coq.
For example, forall x y : N, plus x y = plus y x is an instance of Gallina sentence as well as the body of the plus function. To obtain the term attached to plus_comm use the Print command:
Print plus_comm.
plus_comm =
fun x y : N =>
N_ind (fun x0 : N => plus x0 y = plus y x0)
(eq_ind_r (fun n : N => y = n) eq_refl (plus_0 y))
(fun (x0 : N) (IHx : plus x0 y = plus y x0) =>
eq_ind_r (fun n : N => S n = plus y (S x0))
(eq_ind_r (fun n : N => S (plus y x0) = n) eq_refl (plus_S y x0))
IHx) x
: forall x y : N, plus x y = plus y x
It is not an easy read, but with some experience you'll be able to understand it.
Incidentally, here is how we could have proved the lemma not using tactics:
Definition plus_comm : forall x y : N, plus x y = plus y x :=
fix IH (x y : N) :=
match x return plus x y = plus y x with
| O => eq_sym (plus_0 y)
| S x => eq_ind _ (fun p => S p = plus y (S x)) (eq_sym (plus_S y x)) _ (eq_sym (IH x y))
end.
To explain a few things: fix is the means of defining recursive functions, eq_sym is used to change x = y into y = x, and eq_ind corresponds to the rewrite tactic.
Often in Coq I find myself doing the following: I have the proof goal, for example:
some_constructor a c d = some_constructor b c d
And I really only need to prove a = b because everything else is identical anyway, so I do:
assert (a = b).
Then prove that subgoal, then
rewrite H.
reflexivity.
finishes the proof.
But it seems to just be unnecessary clutter to have those hanging around at the bottom of my proof.
Is there a general strategy in Coq for taking an equality of constructors and splitting it up into an equality of constructor parameters, kinda like a split but for equalities rather than conjunctions.
You can use Coq's searching capabilities:
Search (?X _ = ?X _).
Search (_ _ = _ _).
Among some noise it reveals a lemma
f_equal: forall (A B : Type) (f : A -> B) (x y : A), x = y -> f x = f y
And its siblings for multi-argument equalities: f_equal2 ... f_equal5 (as of Coq version 8.4).
Here is an example:
Inductive silly : Set :=
| some_constructor : nat -> nat -> nat -> silly
| another_constructor : nat -> nat -> silly.
Goal forall x y,
x = 42 ->
y = 6 * 7 ->
some_constructor x 0 1 = some_constructor y 0 1.
intros x y Hx Hy.
apply f_equal3; try reflexivity.
At this point all you need to prove is x = y.
In particular, standard Coq provides the f_equal tactic.
Inductive u : Type := U : nat -> nat -> nat -> u.
Lemma U1 x y z1 z2 : U x y z1 = U x y z2.
f_equal
Also, ssreflect provides a general-purpose congruence tactic congr.
If I'm in Coq and I find myself in a situation with a goal like so:
==================
x = y -> y = x
Is there a tactic that can can take care of this in one swoop? As it is, I'm writing
intros H. rewrite -> H. reflexivity.
But it's a bit clunky.
To "flip" an equality H: x = y you can use symmetry in H. If you want to flip the goal, simply use symmetry.
If you're looking for a single tactic, then the easy tactic handles this one immediately:
Coq < Parameter x y : nat.
x is assumed
y is assumed
Coq < Lemma sym : x = y -> y = x.
1 subgoal
============================
x = y -> y = x
sym < easy.
No more subgoals.
If you take a look at the proof that the easy tactic found, the key part is an application of eq_sym:
sym < Show Proof.
(fun H : x = y => eq_sym H)
The heavier-weight auto tactic will also handle this goal in a single step. For a slightly lower-level proof that produces exactly the same proof term, you can use the symmetry tactic (which also automatically does the necessary intro for you):
sym < Restart.
1 subgoal
============================
x = y -> y = x
sym < symmetry.
1 subgoal
H : x = y
============================
x = y
sym < assumption.
No more subgoals.
sym < Show Proof.
(fun H : x = y => eq_sym H)