Transitivity of -> in Coq

I'm trying to prove the transitivity of -> in Coq's propositions:

Theorem implies_trans : forall P Q R : Prop,
  (P -> Q) -> (Q -> R) -> (P -> R).
Proof.

I wanted to destruct all propositions and simply handle all 8 possibilities with reflexivity. Apparently it's not that simple. Here's what I tried:

Theorem implies_trans : forall P Q R : Prop,
  (P -> Q) -> (Q -> R) -> (P -> R).
Proof.
  intros P Q R H1 H2.
  destruct P. (** Hmmm ... this doesn't work *)
Admitted.

And this is what I get:

1 subgoal
P, Q, R : Prop
H1 : P -> Q
H2 : Q -> R
______________________________________(1/1)
P -> R

followed by this error:

Error: Not an inductive product.

Any help is very much appreciated, thanks!

Coq's logic is not classical logic where propositions are true or false. Instead, it's based in type theory and has an intuitionistic flavor by default.¹ In type theory, you should think of P -> Q being a function from "things of type P" to "things of type Q".²

The usual way to prove a goal of type P -> Q is to use intro or intros to introduce a hypothesis of type P, then use that hypothesis to somehow produce an element of type Q.

For example, we can prove that (P -> Q -> R) -> (Q -> P -> R). In the "implication is a function" interpretation, this could be read as saying that if we have a function that takes P and Q and produces R, then we can define a function that takes Q and P and produces R. This is the same function, but with the arguments swapped.

Definition ArgSwap_1 {P Q R: Prop}: (P -> Q -> R) -> (Q -> P -> R) :=
  fun f q p => f p q.

Using tactics, we can see the types of the individual elements.

Lemma ArgSwap_2 {P Q R: Prop}: (P -> Q -> R) -> (Q -> P -> R).
Proof.
  intro f.
  intros q p.
  exact (f p q).
Qed.

After the intro, we see that f: P -> Q -> R, so f is our function that takes Ps and Qs and produces Rs. After the intros (which introduces multiple terms), we see that q: Q and p: P. The last line (before the Qed.) simply applies the function f to p and q to get something in R.

For your problem, the intros introduces the propositions P, Q and R, as well as H1: P -> Q and H2: Q -> R. We can still introduce one more term of type P though, since the goal is P -> R. Can you see how to use H1 and H2 and an element of P to produce an element of R? Hint: you'll go through Q. Also, remember that H1 and H2 are functions.

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¹ You could add the law of excluded middle as an axiom, which would allow the kind of case analysis you want, but I think it misses the point of Coq.

² If you're wondering, the elements of Prop are still types and have very similar behavior to elements of Set or Type. The only difference is that Prop is "impredicative", which allows propositions to quantify over all propositions. For example forall P: Prop, P -> P is an element of Prop, but forall A: Type, A -> A is an element of the next level up of Type (Type is actually an infinite hierarchy).