I am trying to understand the lemma below.
Why is the ?y2 schematic variable introduced in exI?
And why it is not considered in refl (so: x = x)?
lemma "∀x. ∃y. x = y"
apply(rule allI) (* ⋀x. ∃y. x = y *)
thm exI (* ?P ?x ⟹ ∃x. ?P x *)
apply(rule exI) (* ⋀x. x = ?y2 x *)
thm refl (* ?t = ?t *)
apply(rule refl)
done
UPDATE (because I can't format code in comments):
This is the same lemma with a different proof, using simp.
lemma "∀x. ∃y. x = y"
using [[simp_trace, simp_trace_depth_limit = 20]]
apply (rule allI) (*So that we start from the same problem state. *)
apply (simp only:exI)
done
The trace shows:
[0]Adding rewrite rule "HOL.exI":
?P1 ?x1 ⟹ ∃x. ?P1 x ≡ True
[1]SIMPLIFIER INVOKED ON THE FOLLOWING TERM:
⋀x. ∃y. x = y
[1]Applying instance of rewrite rule "HOL.exI":
?P1 ?x1 ⟹ ∃x. ?P1 x ≡ True
[1]Trying to rewrite:
x = ?x1 ⟹ ∃xa. x = xa ≡ True <-- NOTE: not ?y2 xa or similar!
[2]SIMPLIFIER INVOKED ON THE FOLLOWING TERM:
x = ?x1
[1]SUCCEEDED
∃xa. x = xa ≡ True
So apparently simp and rule handles exI differently. And the remaining question is: what is the mechanical (programmatical) reasoning behind rule's behavior.
When you use rule thm for some fact thm, Isabelle performs higher-order unification of the conclusion of thm with the current goal. If there is a unifier, it is used to instantiate both the goal and the conclusion of the theorem, and then resolution is performed (i.e. the goal is replaced with the assumptions of thm).
This means that:
Schematic variables in the goal can be instantiated by rule through unification
Variables that appear only in the assumptions of thm will not be instantiated by the unification and will therefore remain schematic. That way, you end up with schematic variables in your new goals. Such variables can be seen as existential in some sense, because the conclusion of thm holds if you can prove the assumptions for just one arbitrary value.
In the case of exI, you have ?P ?x ⟹ ∃x. ?P x. When you apply rule exI, the variable ?P is instantiated to λy. x = y, but the variable ?x appears only in the assumptions of exI, so it remains schematic. This means that you can pick any value you want for ?x later on in your proof.
To be more precise, you end up with ⋀x. x = ?y2 x as your goal. You might ask ‘Why not just ⋀x. x = ?y2?’ That would mean that you have to show that x equals some fixed value y2 for all possible values of x. That is obviously not true in general. ⋀x. x = ?y2 x means you have to show that every x equals some y2 that may depend on x – or, equivalently, that there is a function y2 that, when given x, outputs x.
Of course, there is such a function and it is simply the identity function λx. x. That is precisely what ?y2 gets instantiated to when you apply rule refl: the goal x = ?y2 x is unified with the conclusion of refl ?t = ?t and you end up with ?t = x and ?y2 = λx. x, and since refl has no assumptions, this resolution finishes the proof.
I am not entirely sure what you mean with ‘And why it is not considered in refl?’, but I hope that I have answered your questions.
Get a more complete answer from an expert, but I give a short, brief answer to your second part.
The great thing about Isabelle is that it provides many different ways to prove a problem.
Your new question is similar to L.Paulson's comment on FOM: you moved the goal post by switching the question to rule vs. simp:
http://www.cs.nyu.edu/pipermail/fom/2015-October/019312.html
Getting a basic understanding of simp is actually a much easier goal to pursue, or I wouldn't be adding my reponse here.
rule and natural deduction
The use of rule is the use of natural deduction (ND), where most people aren't up to speed on ND. The use of ND requires understanding ND, so questions like your first question can lead to a non-simple answer, because anything informative can't be a one-liner answer, especially due to things like schematic variables (which you asked about), resolution, unification, rewriting, etc.
Do a search on natural deduction and you'll find the standard wiki page about it. There are numerous books on natural deduction, though they get swamped in searches on "logic" due to first-order logic books. A popular book is Logic in Computer Science, 2nd, by Huth and Ryan.
If you study ND, you'll see that exI matches one of the ND rules.
I have yet to take the time to come up to speed on ND, because I keep making progress without having more than a basic understanding of ND.
Sledgehammer, and auto-methods auto, simp, blast, induct, cases, etc., and Sledgehammer's use of some of those, keep me from finding the time to become good with natural decution.
Answer's like M.Eberl's, though not simple explanations, help me absorb a little here and a little there.
Simp, I think of it as simple substitution (rewriting)
The mechanics behind simp is really simple, compared to natural deduction. You define a formula and prove it:
lemma foo [simp]: "left_hand_side = right_hand_side"
In the proof of another theorem, when simp is invoked in one way or another, or foo is unfolded, where there is left_hand_side, it's replaced with right_hand_side. It's just classic mathematical substitution.
I suppose it could also be "rewriting", but I don't know anything about rewriting, other than they talk about it.
There are lots of details about how and whether one should set things up automatically (to prevent looping), like with [simp] or declare foo_def [simp add], but that's just details along the line of normal programming.
Related
I am trying to prove the following basic theorem about the existence of the inverse function of a bijective function (to learn theorem-proving with Isabelle/HOL):
For any set S and its identity map 1_S, α:S→T is bijective iff there
exists a map β: T→S such that βα=1_S and αβ=1_S.
Below is what I have so far after some attempts to define relevant things including functions and their inverses. But I am pretty stuck and couldn't make much progress due to my lack of understanding of Isabelle and/or Isar.
theory Test
imports Main
"HOL.Relation"
begin
lemma bij_iff_ex_identity : "bij_betw f A B ⟷ (∃ g. g∘f = restrict id B ∧ f∘g = restrict id A)"
unfolding bij_betw_def inj_on_def restrict_def iffI
proof
let ?g = "restrict (λ y. (if f x = y then x else undefined)) B"
assume "(∀x∈A. ∀y∈A. f x = f y ⟶ x = y)"
have "?g∘f = restrict id B"
proof
(* cannot prove this *)
end
In above, I try to give an explicit existential witness (i.e. the inverse function g of the original function f). I have several issues about the proof.
whether the concepts are defined right (functions, inverse functions etc.) in Isabelle terms.
how to expand the relevant definitions and then simplify them with function applications. I have followed some Isabelle (2021) examples/tutorials about both the apply-style simp, and structured style Isar proof but couldn't use the Isar proof fluently. Once I started the proof command, I don't know how to simp or move any further.
Isar has the new way of assumes ... shows ... for stating the theorem. Is there similar support for proving iff's (⟷) like the example above? Without it, there is no access to assms etc., and is it necessary to assume everything except the conclusion during the proof.
Can someone help explain how the above existential proof about inverse function can be accomplished?
lemma bij_iff_ex_identity : "bij_betw f A B ⟷ (∃ g. g∘f = restrict id B ∧ f∘g = restrict id A)"
I think this is not exactly what you want an I am doubtful that it is true. g∘f = restrict id B does not mean that g∘f and id are equal on B. It means that the total function g∘f (and there are only total functions in HOL) equals the total function restrict id B. The latter returns id x on x∈B and undefined otherwise. So to make this equality true, g needs to output undefined whenever the input of f is not in B. But how would g know that!
If you want to use restrict, you could write restrict (g∘f) B = restrict id B. But personally, I would rather go for the simpler (∀x∈B. (g∘f) x = x).
So the corrected theorem would be:
lemma bij_iff_ex_identity : "bij_betw f A B ⟷ (∃ g. (∀x∈A. (g∘f) x = x) ∧ (∀y∈B. (f∘g) y = y))"
(Which is still wrong, by the way, as quickcheck tells me in Isabelle/jEdit, see the output window. If A has one element and B is empty, f cannot be a bijection. So the theorem you are attempting is actually mathematically not true. I will not attempt to fix it, but just answer the remaining lines.
unfolding bij_betw_def inj_on_def restrict_def iffI
The iffI here has no effect. Unfolding can only apply theorems of the form A = B (unconditional rewriting rules). iffI is not of that form. (Use thm iffI to see.)
proof
Personally, I don't use the bare form proof but always proof - or proof (some method). Because proof just applies some default method (in this case, equivalent to (rule iffI), so I think it's better to make it explicit. proof - just starts the proof without applying an extra method.
let ?g = "restrict (λ y. (if f x = y then x else undefined)) B"
You have an unbound variable x here. (Note the background color in the IDE.) That is most likely not what you want. Formally, it is allowed, but x will be treated as if it was some arbitrary constant.
Generally, I don't think there is any way to define g in a simple way (i.e., only with quantifiers and function applications and if-then-else). I think the only way to define an inverse (even if you know it exists), is to use the THE operator, because you need to say something like g y is "the" x such that f x = y. (And then later in the proof you will run into a proof obligation that it indeed exists and that it is unique.) See the definition of inv_into in Hilbert_Choice.thy (except it uses SOME not THE). Maybe for starters, try to do the proof just using the existing inv_into constant.
assume "(∀x∈A. ∀y∈A. f x = f y ⟶ x = y)"
All assume commands must have assumptions exactly as the are in the proof goal. You can test whether you wrote it right by just temporarily writing the command show A for A (that's an unprovable goal that would, however, finish the proof, so it tricks Isabelle into checking if it would). If this command does not give an error, you got the assumes right. In your cases, you didn't, it should be (∀x∈A. ∀y∈A. f x = f y ⟶ x = y) ∧ f ' A = B. (' is the backtick symbol here. Markup doesn't let me write it.)
My recommendation: Try the proof with bij instead of bij_betw first. (One direction is in BNF_Fixpoint_Base.o_bij if you want to cheat.)
Once done, you can try to generalize.
I agree with the insightful remarks provided by Dominique Unruh. However, I would like to mention that a theorem that captures the idea underlying the theorem that you are trying to prove already exists in the source code of the main library of Isabelle/HOL. In fact, it exists in at least two different formats: let me name them the traditional Isabelle/HOL format and the canonical FuncSet format. For the former one, see the theorem bij_betw_iff_bijections:
"bij_betw f A B ⟷ (∃g. (∀x ∈ A. f x ∈ B ∧ g(f x) = x) ∧ (∀y ∈ B. g y ∈ A ∧ f(g y) = y))"
The situation is a little bit more complicated with FuncSet. There does not seem to exist a single theorem that captures the idea. However, together, the theorems bij_betwI, bij_betw_imp_funcset and inv_into_funcset are nearly equivalent to the theorem that you are trying to state. Let me provide a sketch of how one could express this theorem in a manner that would be considered reasonably canonical in the FuncSet sense (try to prove it yourself):
lemma bij_betw_iff:
shows "bij_betw f A B ⟷
(
∃g.
(∀x. x∈A ⟶ g (f x) = x) ∧
(∀y. y∈B ⟶ f (g y) = y) ∧
f ∈ A → B ∧
g ∈ B → A
)"
sorry
I would also like to repeat the advice given by Dominique Unruh and provide several side remarks:
My recommendation: Try the proof with bij instead of bij_betw first.
Indeed, this is a very good idea. In general, by trying to restrict the problem to explicitly defined sets A and B, instead of working directly with types, you touched upon a topic that is known as relativization in logic. For a mild layman's introduction see, for example, https://leanprover.github.io/logic_and_proof/first_order_logic.html [1], for a slightly more thorough introduction in the context of set theory see [2, chapter 12]. As you have probably noticed by now, it is not that easy to relativize theorems in Isabelle/HOL and requires additional proof effort.
However, there exists an extension of Isabelle/HOL that allows for the automation of the process of the relativization of theorems. For more information about this extension see the article From Types to Sets by Local Type Definition in Higher-Order Logic by Ondřej Kunčar and Andrei Popescu [3]. There also exists a large scale application example of the framework [4]. Independently, I am working on making this extension more user-friendly and very slowly approaching the final stages in my efforts: see https://gitlab.com/user9716869/tts_extension. Thus, in principle, if you know how to use Types-To-Sets and you accept its axioms, then it is sufficient to prove the theorem with bij, e.g.,
"bij f ⟷ (∃g. (∀x. g (f x) = x) ∧ (∀y. f (g y) = y))",
Then, the theorems like
bij_betw_iff_bijections and bij_betw_iff can be synthesized automatically for free upon a click of a button (almost...).
Finally, for completeness, let me offer my own advice with regard to your queries (although, as I mentioned, I agree with everything stated by Dominique Unruh)
how to expand the relevant definitions and then simplify them with
function applications. I have followed some Isabelle (2021)
examples/tutorials about both the apply-style simp, and structured
style Isar proof but couldn't use the Isar proof fluently. Once I
started the proof command, I don't know how to simp or move any
further.
I believe that the best way to learn what you are trying to learn is by following through the exercises in the book Concrete Semantics by Tobias Nipkow and Gerwin Klein [5]. Additionally, I would also look through A Proof Assistant for Higher-Order Logic by Tobias Nipkow et al [6](it is slightly outdated, but I found it to be useful specifically for learning apply-style scripting/direct rule application). By the way, I have mostly self-taught myself Isabelle from these books without any prior experience in formal methods.
Isar has the new way of assumes ... shows ... for stating the theorem.
Is there similar support for proving iff's (⟷) like the example above?
Without it, there is no access to assms etc., and is it necessary to
assume everything except the conclusion during the proof.
I will make the advice given by Dominique Unruh more explicit: use rule iffI or intro iffI for this.
Edit. When you use rule iffI (or similar) to start your structured Isar proof, you need to state your assumptions explicitly for every subgoal (using the assume ... show ... paradigm). However, there is a tool that can generate such boilerplate Isar code automatically. It is called Sketch-and-Explore and you can find it in the directory HOL/ex of the main library of Isabelle/HOL. In this case, all you need to do is to type sketch(rule iffI) and the assume/show paradigm will be generated automatically for every subgoal.
References
Avigad J, Lewis RY, and van Doorn F. Logic and Proof.
Jech T. Set theory. 3rd ed. Heidelberg: Springer; 2006. (Pure and applied mathematics, a series of monographs and textbooks).
Kunčar O, Popescu A. From Types to Sets by Local Type Definition in Higher-Order Logic. Journal of Automated Reasoning. 2019;62(2):237–60.
Immler F, Zhan B. Smooth Manifolds and Types to Sets for Linear Algebra in Isabelle/HOL. In: 8th ACM SIGPLAN International Conference on Certified Programs and Proofs. New York: ACM; 2019. p. 65–77. (CPP 2019).
Nipkow T, Klein G. Concrete Semantics with Isabelle/HOL. Heidelberg: Springer-Verlag; 2017. (http://concrete-semantics.org/)
Nipkow T, Paulson LC, Wenzel M. A Proof Assistant for Higher-Order Logic. Heidelberg: Springer-Verlag; 2017.
I sometimes find it hard to use Isabelle because I cannot have a "print command" like in normal programming.
For example, I want to see what ?thesis. The concrete semantics book says:
The unknown ?thesis is implicitly matched against any goal stated by lemma or show. Here is a typical example:
My silly sample FOL proof is:
lemma
assumes "(∃ x. ∀ y. x ≤ y)"
shows "(∀x. ∃ y. y ≤ x)"
proof (rule allI)
show ?thesis
but I get the error:
proof (state)
goal (1 subgoal):
1. ⋀x. ∃y. y ≤ x
Failed to refine any pending goal
Local statement fails to refine any pending goal
Failed attempt to solve goal by exported rule:
∀x. ∃y. y ≤ x
but I do know why.
I expected
?thesis === ⋀x. ∃y. y ≤ x
since my proof state is:
proof (state)
goal (1 subgoal):
1. ⋀x. ∃y. y ≤ x
Why can't I print ?thesis?
It's really annoying to have to write the statement I'm trying to proof if it's obvious. Perhaps it's meant to be explicit but in the examples in chapter 5 they get away with using ?thesis in:
lemma fixes a b :: int assumes "b dvd (a+b)" shows "b dvd a" proof −
have "∃k′. a = b∗k′" if asm: "a+b = b∗k" for k proof
show "a = b∗(k − 1)" using asm by(simp add: algebra_simps) qed
then show ?thesis using assms by(auto simp add: dvd_def ) qed
but whenever I try to use ?thesis I always fail.
Why is it?
Note that this does work:
lemma
assumes "(∃ x. ∀ y. x ≤ y)"
shows "(∀x. ∃ y. y ≤ x)"
proof (rule allI)
show "⋀x. ∃y. y ≤ x" proof -
but I thought ?thesis was there to avoid this.
Also, thm ?thesis didn't work either.
Another example is when I use:
let ?ys = take k1 xs
but I can't print ?ys value.
TODO:
why doesn't:
lemma "length(tl xs) = length xs - 1"
thm (cases xs)
show anything? (same if your replaces cases with induction).
You can find ?theorem and others in the print context window:
As for why ?thesis doesn't work, by applying the introduction rule proof (rule allI) you are changing the goal, so it no longer matches ?thesis. The example in the book uses proof- which prevents Isabelle from applying any introduction rule.
It seems I asked a very similar question worth pointing to: What is the best way to search through general definitions, theorems, functions, etc for Isabelle?
But here is a list of thing's I've learned so far:
thm: seems to work for definition, lemmas and functions. For definition do name_def for a definition with name name. For functions do thm f.simps for all definitions in the function. For a single one do thm f.simps(1) for the first one. For lemmas do thm lemma_name or thm impI or HOL.mp etc.
term: for terms do term term_name e.g. in isar term ?thesis or term this
print_theorems: if you place this after a definition or a function it shows all the theorems defined for those! It's amazing.
print... I just noticed in jedit if you let the auto complete show you the rest for print it has a bunch of options! Probably useful!
Search engine for Isabelle: https://search.isabelle.in.tum.de/
You can use Query (TODO: improve this)
TODO: how to find good way to display stuff about tactics.
I plan to update this as I learn all the ways to debug in Isabelle.
I want to prove that if one implication is true (b --> c), then it follows that some other implication is also true (a --> c), given that there is an appropriate relation between the a and b.
Here is a concrete toy example that I'm trying to prove:
theory Prop imports Main
begin
lemma
fixes func1 :: "'a => 'a"
and func2 :: "'a => 'a"
and func3 :: "'a => 'a"
assumes "∀ x y. func1 x = func1 y --> func3 x = func3 y"
and "∀ x. func1 x = func2 x"
shows "∀ x y. func2 x = func2 y --> func3 x = func3 y"
proof -
from assms show ?thesis by auto
qed
end
The prove does not succed, Isabelle 2018 keeps running and the "by auto" part is colored purple. How should I go about proving this kind of lemmas?
It seems that your problem makes the simplifier (which is part of auto) loop. I don't really see why, but these things do happen occasionally.
When this happens, it can sometimes help to run try0 (which just tries a couple of common automatic proof methods and returns those that succeed) or sledgehammer (which tries to translate the problem into a simpler form and give it to external provers; if they can prove it, it then tries to translate the proof back to Isabelle).
In this case, both try0 and sledgehammer find that a simple apply metis can do the job. Methods like auto and simp do lots of things, including, most notably, ‘stupid’ rewriting with a pre-defined set of rules. metis is a bit more clever in what it does, but you need to manually give it every fact that it is supposed to use, and it is less adapted to Isabelle/HOL.
However, since this problem is simple first-order logic, metis can easily solve them by itself with no explicitly given facts, and it manages to avoid whatever pitfall causes auto and simp to diverge.
Suppose some lemma, lets call it an_equation, proves that equation f(n)=n*n+1 holds for all odd natural numbers n (and f is some previously defined function).
How can I instantiate this lemma to concrete values of n, so that I can prove, say f(5)=5*5+1?
(There is the lemmas keyword, with which I can prove lemmas inst = an_equation[where n=5, simplified], but this is not quite what I what. What I want is
lemma inst_new : "f(5) = 5*5+1"
but as there were scarcely any example around in the usual documentation thai I consult, I couldn't figure out how to prove this.)
You can instantiate free variables in your theorem with the of and where attributes, e.g. an_equation[of 5] or an_equation[where n = 5]. You can also instantiate its assumptions, e.g. if you have a theorem called foo of the form P x ⟹ Q x and you have a theorem called bar of the form P 5, you can do foo[OF bar] to get the theorem Q 5.
You can inspect these instantiated with thm (e.g. thm foo[of 5]) and use them in a proof with using, from, etc.
Note that proof methods also instantiate theorems whenever needed, e.g. if you have, as above, the theorem foo stating that P n ⟹ Q n and you have the goal Q 5, you can do this:
lemma "Q 5"
apply (rule foo)
Then the goal state will have one subgoal, namely P 5.
So, in your case, by (rule an_equation) should do the trick. Something like by (simp add: an_equation) or using an_equation by simp or using an_equation by blast would probably also work, but might be slightly less robust in general since the simplifier can do other stuff first and then the rule might not apply anymore.
I'm an Isabelle newbie, and I'm a little (actually, a lot) confused about the relationship between ⋀ and ∀, and between ⟹ and ⟶.
I have the following goal (which is a highly simplified version of something that I've ended up with in a real proof):
⟦⋀x. P x ⟹ P z; P y⟧ ⟹ P z
which I want to prove by specialising x with y to get ⟦P y ⟹ P z; P y⟧ ⟹ P z, and then using modus ponens. This works for proving the very similar-looking:
⟦∀x. P x ⟶ P z; P y⟧ ⟹ P z
but I can't get it to work for the goal above.
Is there a way of converting the former goal into the latter? If not, is this because they are logically different statements, in which case can someone help me understand the difference?
That the two premises !!x. P x ==> P y and ALL x. P x --> P y are logically equivalent can be shown by the following proof
lemma
"(⋀x. P x ⟹ P y) ≡ (Trueprop (∀x. P x ⟶ P y))"
by (simp add: atomize_imp atomize_all)
When I tried the same kind of reasoning for your example proof I ran into a problem however. I intended to do the following proof
lemma
"⟦⋀x. P x ⟹ P z; P y⟧ ⟹ P z"
apply (subst (asm) atomize_imp)
apply (unfold atomize_all)
apply (drule spec [of _ y])
apply (erule rev_mp)
apply assumption
done
but at unfold atomize_all I get
Failed to apply proof method:
When trying to explicitly instantiate the lemma I get a more clear error message, i.e.,
apply (unfold atomize_all [of "λx. P x ⟶ P z"])
yields
Type unification failed: Variable 'a::{} not of sort type
This I find strange, since as far as I know every type variable should be of sort type. We can solve this issue by adding an explicit sort constraint:
lemma
"⟦⋀x::_::type. P x ⟹ P z; P y⟧ ⟹ P z"
Then the proof works as shown above.
Cutting a long story short. I usually work with Isar structured proofs instead of apply scripts. Then such issues are often avoided. For your statement I would actually do
lemma
"⟦⋀x. P x ⟹ P z; P y⟧ ⟹ P z"
proof -
assume *: "⋀x. P x ⟹ P z"
and **: "P y"
from * [OF **] show ?thesis .
qed
Or maybe more idiomatic
lemma
assumes *: "⋀x. P x ⟹ P z"
and **: "P y"
shows "P z"
using * [OF **] .
C.Sternagel answered your title question "How?", which satisfied your last sentence, but I go ahead and fill in some details based on his answer, to try to "help [you] understand the difference".
It can be confusing that there is ==> and -->, meta-implication and HOL-implication, and that they both have the properties of logical implication. (I don't say much about !! and !, meta-all and HOL-all, because what's said about ==> and --> can be mostly be transferred to them.)
(NOTE: I convert graphical characters to equivalent ASCII when I can, to make sure they display correctly in all browsers.)
First, I give some references:
[1] Isabelle/Isar Reference manual.
[2] HOL/HOL.thy
[3] Logic in Computer Science, by Huth and Ryan
[4] Wiki sequent entry.
[5] Wiki intuitionistic logic entry.
If you understand a few basics, there's nothing that confusing about the fact that there is both ==> and -->. Much of the confusion departs, and what's left is just the work of digging through the details about what particular source statements mean, such as the formula of C.Sternagel's first lemma.
"(!!x. P x ==> P y) == (Trueprop (!x. P x --> P y))"
C.Sternagel stopped taking the time to give me important answers, but the formula he gives you above is similar to one he gave me a while ago, to convince me that all free variables in a formula are universally quantified.
Short answer: The difference between ==> and --> is that ==> (somewhat) plays the part of the turnstile symbol, |-, of a non-generalized sequent in which there is only one conclusion on the right-hand side. That is, ==>, the meta-logic implication operator of Isabelle/Pure, is used to define the Isabelle/HOL implication object-logic operator -->, as shown by impI in the following axiomatization in HOL.thy [2].
(*line 56*)
typedecl bool
judgment
Trueprop :: "bool => prop"
(*line 166*)
axiomatization where
impI: "(P ==> Q) ==> P-->Q" and
mp: "[| P-->Q; P |] ==> Q" and
iff: "(P-->Q) --> (Q-->P) --> (P=Q)" and
True_or_False: "(P=True) | (P=False)"
Above, I show the definition of three other axioms: mp (modus ponuns), iff, and True_or_False (law of excluded middle). I do that to repeatedly show how ==> is used to define the axioms and operators of the HOL logic. I also threw in the judgement to show that some of the sequent vocabulary is used in the language Isar.
I also show the axiom True_or_False to show that the Isabelle/HOL logic has an axiom which Isabelle/Pure doesn't have, the law of excluded middle [5]. This is huge in answering your question "what is the difference?"
It was a recent answer by A.Lochbihler that finally gave meaning, for me, to "intuitionistic" [5]. I had repeatedly seen "intuitionistic" in the Isabelle literature, but it didn't sink in.
If you can understand the differences in the next source, then you can see that there's a big difference between ==> and -->, and between types prop and bool, where prop is the type of meta-logic propositions, as opposed to bool, which is the type of the HOL logic proposition. In the HOL object-logic, False implies any proposition Q::bool. However, False::bool doesn't imply any proposition Q::prop.
The type prop is a big part of the meta-logic team !!, ==>, and ==.
theorem "(!!P. P::bool) == Trueprop (False::bool)"
by(rule equal_intr_rule, auto)
theorem HOL_False_meta_implies_any_prop_Q:
"(!!P. P::bool) ==> PROP Q"
(*Currently, trying by(auto) will hang my machine due to blast, which is know
to be a problem, and supposedly is fixed in the current repository. With
`Auto methods` on in the options, it tries `auto`, thus it will hang it.*)
oops
theorem HOL_False_meta_implies_any_bool_Q:
"(!!P. P::bool) ==> Q::bool"
by(rule meta_allE)
theorem HOL_False_obj_implies_any_bool_Q:
"(!P. P::bool) --> Q::bool"
by(auto)
When you understand that Isabelle/Pure meta-logic ==> is used to define the HOL logic, and other differences, such as that the meta-logic is weaker because of no excluded middle, then you understand that there are significant differences between the meta-operators, !!, ==>, and ==, in comparison to the HOL object-logic operators, !, -->, and =.
From here, I put in more details, partly to convince any expert that I'm not totally abusing the word sequent, where my use here is based primarily on how it's used in reference [3, Huth and Ryan].
Attempting to not write a book
I throw in some quotes and references to show that there's a relationship between sequents and ==>.
From my research, I can't see that the word "sequent" is standardized. As far as I can tell, in [3.pg 5], Huth and Ryan use "sequent" to mean a sequent which has only has one conclusion on the right-hand side.
...This intention we denote by
phi1, phi2, ..., phiN |- psi
This expression is called a sequent; it is valid if a proof can be found.
A more narrow definition of sequent, in which the right-hand side has only one conclusion, matches up very nicely with the use of ==>.
We can blame L.Paulson for confusing us by separating the meta-logic from the object-logic, though we can thank him for giving us a larger logical playground.
Maybe to keep from clashing with the common definition of a sequent, as in [4, Wiki], he uses the phrase natural deduction sequent calculus in various places in the literature. In any case, the use of ==> is completely related to implementing natural deduction rules in the logic of Isabelle/HOL.
Even with generalized sequents, L.Paulson prefers the ==> notation:
Logic and Proof course 2012-13
Course materials: see slides for his generalized sequent calculus notation
You asked about differences. I throw in some source related to C.Sternagel's answer, along with the impI axiomatization again:
(*line 166*)
axiomatization where
impI: "(P ==> Q) ==> P-->Q"
(*706*)
lemma --"atomize_all [atomize]:"
"(!!x. P x) == Trueprop (ALL x. P x)"
by(rule atomize_all)
(*715*)
lemma --"atomize_imp [atomize]:"
"(A ==> B) == Trueprop (A --> B)"
by(rule atomize_imp)
(*line 304*)
lemma --"allI:"
assumes "!!x::'a. P(x)"
shows "ALL x. P(x)"
by(auto simp only: assms allI
I put impI in structured proof format:
lemma impI_again:
assumes "P ==> Q"
shows "P --> Q"
by(simp add: assms)
Now, consider ==> to be the use of the sequent turnstile, and shows to be the sequent notation horizontal bar, then you have the following sequent:
P |- Q
-------
P --> Q
This is the natural deduction implication introduction rule, as the axiom name says, impI (Cornell Lecture 15).
The Big Guys have been on top of all of this for a long time. See [1, Section 2.1, page 27] for an overview of !!, ==>, and ==. In particular, it says
The Pure logic [38, 39] is an intuitionistic fragment of higher-order logic
[13]. In type-theoretic parlance, there are three levels of lambda-calculus with
corresponding arrows =>/!!/==>`...
One general significance of the statement is that in the use of Isabelle/HOL, you are using two logics, a meta-logic and an object-logic, where those two terms come from L.Paulson, and where "intuitionistic" is a key defining point of the meta-logic.
See also [1, Section 9.4.1, Simulating sequents by natural deduction, pg 206]. According to M.Wenzel on the IsaUsersList, L.Paulson wrote this section. On page 205, Paulson first takes the definition of a sequent to be the generalized definition. On page 206, he then shows how you can line up one type of sequent with the use of ==>, which is by negating every proposition on the right-hand side of a sequent, except for one of them.
That, by all appearances, is a horn clause, which I know nothing about.
It seems obvious to me that using ==> is the use of a limited form of sequents. In any event, that's how I think of it, and thinking that way has given me an understanding of the differences between ==> and -->, along with the fact that the meta-logic has no excluded middle.
If A.Lochbhiler wouldn't have pointed out the absence of an excluded middle, I wouldn't have seen an important difference of what's possible with ==>, and what's possible with -->.
Maybe C.Sternagel will start back again to give me some of his important answers.
Please pardon the long answer.
Others have already explained some of the reasons behind the difference between meta-logic and logic, but missed the simple tactic apply atomize:
lemma "⟦⋀(x::'a). P x ⟹ P z ; P y⟧ ⟹ P z"
apply atomize
which yields the goal:
⟦ ∀x. P x ⟶ P z; P y ⟧ ⟹ P z
as desired.
(The additional type constraint ⋀(x::'a) is required for the reasons mentioned by chris.)
There is a lot of text already, so just a few brief notes:
Isabelle/Pure is minimal-higher order logic with the main connectives ⋀ and ⟹ to lay out Natural Deduction rules in a declarative way. The system knows how to compose them by basic means, e.g. in Isar proofs, proof methods like rule, attributes like OF.
Isabelle/HOL is full higher-order logic, with the full set of predicate logic connectives, e.g. ∀ ∃ ∧ ∨ ¬ ⟶ ⟷, and much more library material. Canonical introduction rules like allI, allE, exI, exE etc. for these connectives explain formally how the reasoning works wrt. the Pure framework. HOL ∀ and ⟶ somehow correspond to Pure ⋀ and ⟹, but they are of different category and should not be thrown into the same box.
Note that apart from the basic thm command to print such theorems, it occasionally helps to use print_statement to get an Isar reading of these Natural Deduction reasoning forms.