Proof Pitfalls

Induction

Introducing too much.

https://softwarefoundations.cis.upenn.edu/draft/lf-current/Tactics.html#lab139

By introducing a variable you are saying for a “particular” instance of that variable. Not for all such instances of the variable.

Thus, when introducing more than necessary prior to induction you may unintentionally weaken the induction hypothesis that you will get, making it impossible to prove your goal.

Destruct

Forgetting things

When you destruct a compound expression you lose what the original equation was. For instance:

destruct (beq_nat 3 n).

will give you two subgoals. One where (beq_nat 3 n) is replaced with true, and one where it is replaced with false.

Sometimes, however, you actually need the fact that (beq_nat 3 n) = true in that branch of the proof, or that (beq_nat 3 n) = false in the other branch of the proof. You can keep this information with:

destruct (beq_nat 3 n) eqn:Hbeq3.

Which will introduce in the context a hypothesis Hbeq3 which will be beq_nat 3 n = true in the true branch of the proof, and beq_nat 3 n = false in the false branch.

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Coq Cheat Sheet

intros

Introduces variables and hypothesis. This takes them out of the goal and puts them in the context.

reflexivity

Does reflexivity. If a goal has x = y it will try to simplify (by normalization) both x and y. If their normal forms are syntactically identical, then this tactic will succeed and the goal will be completed.

E.g.,

1 + 1 = 2

Reflexivity will solve this because 1 + 1 will evaluate to 2, and 2 = 2 is obviously true because each side of the equality is syntactically identical.

simpl

Attempts to simplify the goal. This essentially just evaluates to weak head normal form. Useful for seeing next steps in a proof (hard to unwind definitions in your head sometimes), and it can be used to facilitate rewrites.

1 + 1 = 2

simpl will transform this to 2 = 2.

rewrite

When we have x = y it can be used to replace x with y in a goal, or vice versa.

H : x = y
=========
x = y

Then rewrite H will leave us with the goal y = y, and rewrite <- H will leave us with x = x.

apply

Used to solve a goal by applying a theorem which has an identical conclusion to the current goal. Any hypothesis of the theorem will be added to the context.

This tactic can also be used on hypothesis in the context which then matches on the hypothesis of the theorem being applied and gives you a hypothesis in the context matching the goal of the theorem being applied.

H : x = y
=========
x = y

apply H will solve the goal.

H : x = y -> y = z
H0 : x = y
==================
y = z

You could solve this in two ways with apply:

  • apply H will use the theorem H to show that y = z if x = y, so it leaves you with x = y in your goal. This can then be solved by applying H0.
  • apply H in H0 will use H : x = y -> y = z to transform H0 : x = y to y = z. After this we can apply H0 to our goal.

symmetry

Reverses an equality. Using symmetry will flip x = y to y = x.

This is useful for when you need to apply a theorem, but the goal is in a different order than the theorem.

destruct

The destruct tactic is used to perform case analysis in Coq. It will break a possible value into all cases for that type (one for each constructor of the type). This gives you multiple goals to prove; one for each constructor.

This can be used on variables, or compound expressions.

When using destruct on a compound expression it is also possible to save the original expression.

destruct (f (x + y)) eqn:Hfxy

induction

The induction tactic is very similar to destruct, except that it brings an induction hypothesis into the context for recursive data types.

inversion

inversion provides reasoning with constructors taking into account the fact that constructors are injective and disjoint.

  • Injective: for a constructor A, A x = A y means that x = y as well.
  • Disjoint: If you have two constructors A and B, then you know that A does not equal B

Thus the inversion tactic has several uses.

Injective usage

H : A x = A y
=============
x = y

When using inversion in H this gives us a new hypothesis, H1.

H : A x = A y
H1 : x = y
=============
y = y

It will also perform rewrites with the new hypothesis automatically, so our goal changed to y = y as well, since it rewrote x with y using the new hypothesis H1.

Inversion will apply this injective reasoning across multiple arguments in a constructor, and even recursively. So, if you have lists of three items that are equal you will get a hypothesis representing the equality of each item in the first list, with the equivalent item in the second list.

We can name the equations as well.

inversion in H as [Hxy] will give:

H : A x = A y
Hxy : x = y
=============
y = y

Disjoint usage

If we have values constructed with two different constructors A and B, then we know that the values must be different. So if we have a hypothesis in the context with disjoint constructors, like so:

H: A x = B y
============
false = true

We can use inversion H which will conclude that H is a false hypothesis, and since we have a contradiction in our set of assumptions we may conclude the goal via the principle of explosion.

generalize dependent

The generalize dependent tactic can be used to place a variable in the context back into the goal. This can be useful when you only want to introduce certain variables, like when you want to keep your induction hypothesis strong.

unfold

This tactic is used to expand a definition.

Definition square n := n * n
square (n * m) = square n * square m

unfold square will yield

(n * m) * (n * m) = (n * n) * (m * m)

which will actually display as

n * m * (n * m) = n * n * (m * m)

due to the associativity of the operators.

assert

This tactic will introduce a hypothesis into the context, and then split the goal into two subgoals. The first subgoal is to prove that this hypothesis is true, and the second subgoal is the original goal.

replace

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