I want to proof this lemma in Coq:
a : Type
b : Type
f : a -> b
g : a -> b
h : a -> b
______________________________________(1/1)
(forall x : a, f x = g x) ->
(forall x : a, g x = h x) -> forall x : a, f x = h x
I know that Coq.Relations.Relation_Definitions defines transitivity for relations:
Definition transitive : Prop := forall x y z:A, R x y -> R y z -> R x z.
Simply using the proof tactic apply transitivity obviously fails. How can I apply the transitivity lemma to the goal above?
The transitivity tactic requires an argument, which is the intermediate term that you want to introduce into the equality. First call intros (that's almost always the first thing to do in a proof) to have the hypotheses nicely in the environment. Then you can say transitivity (g x) and you're left with two immediate applications of an assumption.
intros.
transitivity (g x); auto.
You can also make Coq guess which intermediate term to use. This doesn't always work, because sometimes Coq finds a candidate that doesn't work out in the end, but this case is simple enough and works immediately. The lemma that transitivity applies is eq_trans; use eapply eq_trans to leave a subterm open (?). The first eauto chooses a subterm that works for the first branch of the proof, and here it also works in the second branch of the proof.
intros.
eapply eq_trans.
eauto.
eauto.
This can be abbreviated as intros; eapply eq_trans; eauto. It can even be abbreviated further to
eauto using eq_trans.
eq_trans isn't in the default hint database because it often leads down an unsuccessful branch.
Ok, I was on the wrong track. Here is the proof of the lemma:
Lemma fun_trans : forall (a b:Type) (f g h:a->b),
(forall (x:a), f x = g x) ->
(forall (x:a), g x = h x) ->
(forall (x:a), f x = h x).
Proof.
intros a b f g h f_g g_h x.
rewrite f_g.
rewrite g_h.
trivial.
Qed.
Related
I have started playing around with Cubical Agda. Last thing I tried doing was building the type of integers (assuming the type of naturals is already defined) in a way similar to how it is done in classical mathematics (see the construction of integers on wikipedia). This is
data dInt : Set where
_⊝_ : ℕ → ℕ → dInt
canc : ∀ a b c d → a + d ≡ b + c → a ⊝ b ≡ c ⊝ d
trunc : isSet (dInt)
After doing that, I wanted to define addition
_++_ : dInt → dInt → dInt
(x ⊝ z) ++ (u ⊝ v) = (x + u) ⊝ (z + v)
(x ⊝ z) ++ canc a b c d u i = canc (x + a) (z + b) (x + c) (z + d) {! !} i
...
I am now stuck on the part between the two braces. A term of type x + a + (z + d) ≡ z + b + (x + c) is asked. Not wanting to prove this by hand, I wanted to use the ring solver made in Cubical Agda. But I could never manage to make it work, even trying to set it up for simple ring equalities like x + x + x ≡ 3 * x.
How can I make it work ? Is there a minimal example to make it work for naturals ? There is a file NatExamples.agda in the library, but it makes you have to rewrite your equalities in a convoluted way.
You can see how the solver for natural numbers is supposed to be used in this file in the cubical library:
Cubical/Tactics/NatSolver/Examples.agda
Note that this solver is different from the solver for commutative rings, which is designed for proving equations in abstract rings and is explained here:
Cubical/Tactics/CommRingSolver/Examples.agda
However, if I read your problem correctly, the equality you want to prove requires the use of other propositional equalities in Nat. This is not supported by any solver in the cubical library (as far as I know, also the standard library doesn't support it). But of course, you can use the solver for all the steps that don't use other equalities.
Just in case you didn't spot this: here is a definition of the integers in math-style using the SetQuotients of the cubical library. SetQuotients help you to avoid the work related to your third constructor trunc. This means you basically just need to show some constructions are well defined as you would in 'normal' math.
I've successfully used the ring solver for exactly the same problem: defining Int as a quotient of ℕ ⨯ ℕ. You can find the complete file here, the relevant parts are the following:
Non-cubical propositional equality to path equality:
open import Cubical.Core.Prelude renaming (_+_ to _+̂_)
open import Relation.Binary.PropositionalEquality renaming (refl to prefl; _≡_ to _=̂_) using ()
fromPropEq : ∀ {ℓ A} {x y : A} → _=̂_ {ℓ} {A} x y → x ≡ y
fromPropEq prefl = refl
An example of using the ring solver:
open import Function using (_$_)
import Data.Nat.Solver
open Data.Nat.Solver.+-*-Solver
using (prove; solve; _:=_; con; var; _:+_; _:*_; :-_; _:-_)
reorder : ∀ x y a b → (x +̂ a) +̂ (y +̂ b) ≡ (x +̂ y) +̂ (a +̂ b)
reorder x y a b = fromPropEq $ solve 4 (λ x y a b → (x :+ a) :+ (y :+ b) := (x :+ y) :+ (a :+ b)) prefl x y a b
So here, even though the ring solver gives us a proof of _=̂_, we can use _=̂_'s K and _≡_'s reflexivity to turn that into a path equality which can be used further downstream to e.g. prove that Int addition is representative-invariant.
I'm trying to use Isabelle to do auto-prove. However, I got a problem of specifying formulas in Isabelle. For example, I have a formulas like this
Then, I define sets and use big_wedge and big_vee symbols in Isabelle as follows:
And the result is "Inner lexical error⌂ Failed to parse prop".
Could you explain what is wrong here, please?
Thank you very much.
Not all symbols shown in Isabelle/jEdit's Symbol tabs have a meaning. These are the symbols you can use in your code.
Based on the corresponding code for sums, I started the setup, but I did not finish it (in particular, the syntax ⋀t!=l. P t is not supported).
context comm_monoid_add
begin
sublocale bigvee: comm_monoid_set HOL.disj False
defines bigvee = bigvee.F and bigvee' = bigvee.G
by standard auto
abbreviation bigvee'' :: ‹bool set ⇒ bool› ("⋁")
where "⋁ ≡ bigvee (λx. x)"
sublocale bigwedge: comm_monoid_set HOL.conj True
defines bigwedge = bigwedge.F and bigwedge' = bigwedge.G
by standard auto
abbreviation bigwedge'' :: ‹bool set ⇒ bool› ("⋀")
where "⋀ ≡ bigwedge (λx. x)"
end
syntax
"_bigwedge" :: "pttrn ⇒ 'a set ⇒ 'b ⇒ 'b::comm_monoid_add" ("(2⋀(_/∈_)./ _)" [0, 51, 10] 10)
translations ― ‹Beware of argument permutation!›
"⋀i∈A. b" ⇌ "CONST bigwedge (λi. b) A"
syntax
"_bigvee" :: "pttrn ⇒ 'a set ⇒ 'b ⇒ 'b::comm_monoid_add" ("(2⋁(_/∈_)./ _)" [0, 51, 10] 10)
translations ― ‹Beware of argument permutation!›
"⋁i∈A. b" ⇌ "CONST bigvee (λi. b) A"
instantiation bool :: comm_monoid_add
begin
definition zero_bool where
[simp]: ‹zero_bool = False›
definition plus_bool where
[simp]: ‹plus_bool = (∨)›
instance
by standard auto
end
thm bigvee_def
lemma ‹finite A ⟹ (⋁i∈A. f i) ⟷ (∃i ∈ A. f i)›
apply (induction rule: finite_induct)
apply (auto simp: )
done
lemma ‹finite A ⟹ (⋀i∈A. f i) ⟷ A = {} ∨ (∀i ∈ A. f i)›
apply (induction rule: finite_induct)
apply (auto simp: )[2]
done
lemma ‹infinite A ⟹ (⋀i∈A. f i) ⟷ True›
by auto
lemma test1:
‹(⋀j∈L. ⋀u∈U. ⋀t∈T. ⋀l∈L. ⋀l⇩1∈L⇩1. ¬P j u t l⇩1) ∨
(⋁i∈I. ⋁v∈V. ⋀k∈K. ⋁h∈H. Q i ∨ k h) ⟹
(⋁i∈I. ⋁v∈V. ⋀k∈K. ⋁h∈H. Q i ∨ k h) ∨ (⋀j∈J. ⋀u∈U. ⋀t∈T. ⋀l⇩1∈L⇩1. ¬P j u t l⇩1)›
apply auto
The full setup is possible. But I am not certain that this is a good idea... You will need a lot of lemmas to make things work nicely and I am not certain the behaviour for infinite sets is the right one.
So I am still new to coq and MSets are giving me some issues. Here are two functions to compute whether an element is in a list or set, please let me know if you think the set_contains definition is correct or if there is a better way to do it. Thanks for any help.
Require Import MSets ZArith.
Module mset := MSetAVL.Make Positive_as_OT.
Notation pos_set := mset.t.
Definition set_contains (x : positive) (s : pos_set) :=
mset.mem x s.
Fixpoint list_contains (x : positive) (l : list positive) : bool :=
match l with
| nil => false
| y :: l' =>
if Pos.eqb x y then true
else nodelist_contains x l'
end.
Lemma nodelist_nodeset_contains :
forall x (s : pos_set),
(nodelist_contains x (mset.elements s)) = (nodeset_contains x s).
Proof.
induction s.
destruct list_contains.
destruct set_contains.
auto.
It seems that set_contains evaluates to true at the base case after the destructs and i'm not sure why. Would the set not be mset.empty during that stage of the proof?
I also do not know how to work with the mset.In, I have trouble with the base case of this proof, obviously I have the same problem. I want to eventually state:
Lemma nodelist_containsP :
forall x (l : pos_set),
reflect (mset.In x l) (nodeset_contains x l).
In case anyone is interested here is how I did this proof.
intros.
apply iff_reflect.
unfold nodeset_contains.
symmetry.
apply mset.mem_spec.
Qed.
list_contains and set_contains are functions so it does not make sense to try to destruct them. Coq tries to infer what you meant and guesses that you want to case on the value of an expression starting with list_contains and set_contains respectively.
This is not what you want. What you want is to observe the behaviour of the two functions on the same input. And you can do so by inspecting it.
This should send you in the right direction:
destruct s as [mset mset_isok].
induction mset.
+ unfold set_contains, mset.mem.
simpl.
reflexivity.
+ unfold list_contains, set_contains, mset.mem.
simpl.
I am reading the book Introduction to Mathematics Philosophy by B.Russell and trying to formalize the definitions. Whereas I got stuck on proving the equivalence between the two definitions of similarity posed in the book.
Here are the text quoted from the book. (context)
1) Defining similarity directly:
We may define two relations P and Q as “similar,” or as having
“likeness,” when there is a one-one relation S whose domain is the
field of P and whose converse domain is the field of Q, and which is
such that, if one term has the relation P to another, the correlate of
the one has the relation Q to the correlate of the other, and vice
versa.
Here's my comprehension of the above text:
Inductive similar {X} (P : relation X) (Q : relation X) : Prop :=
| similar_intro : forall (S : relation X),
one_one S ->
(forall x, field P x <-> domain S x) ->
(forall x y z w, P x y -> S x z -> S y w -> Q z w) ->
(forall x y z w, Q z w -> S x z -> S y w -> P x y) ->
similar P Q.
2) Defining similarity through the concept of 'correlator':
A relation S is said to be a “correlator” or an “ordinal correlator”
of two relations P and Q if S is one-one, has the field of Q for its
converse domain, and is such that P is the relative product of S and Q
and the converse of S.
Two relations P and Q are said to be “similar,” or to have “likeness,”
when there is at least one correlator of P and Q.
My definition to this is:
Inductive correlator {X} (P Q : relation X) : relation X -> Prop :=
| correlator_intro : forall (S : relation X),
one_one S ->
(forall x, field P x <-> domain S x) ->
(forall x y, relative_product (relative_product S Q) (converse S) x y <-> P x y) ->
correlator P Q S.
Inductive similar' {X} (P Q : relation X) : Prop :=
| similar'_intro : forall S, correlator P Q S -> similar' P Q.
But I couldn't prove that similar is equivalent to similar', where did I make the mistake? Thanks a lot.
Is there an extensible, efficient way to write existential statements in Haskell without implementing an embedded logic programming language? Oftentimes when I'm implementing algorithms, I want to express existentially quantified first-order statements like
∃x.∃y.x,y ∈ xs ∧ x ≠ y ∧ p x y
where ∈ is overloaded on lists. If I'm in a hurry, I might write perspicuous code that looks like
find p [] = False
find p (x:xs) = any (\y -> x /= y && (p x y || p y x)) xs || find p xs
or
find p xs = or [ x /= y && (p x y || p y x) | x <- xs, y <- xs]
But this approach doesn't generalize well to queries returning values or predicates or functions of multiple arities. For instance, even a simple statement like
∃x.∃y.x,y,z ∈ xs ∧ x ≠ y ≠ z ∧ f x y z = g x y z
requires writing another search procedure. And this means a considerable amount of boilerplate code. Of course, languages like Curry or Prolog that implement narrowing or a resolution engine allow the programmer to write statements like:
find(p,xs,z) = x ∈ xs & y ∈ xs & x =/= y & f x y =:= g x y =:= z
to abuse the notation considerably, which performs both a search and returns a value. This problem arises often when implementing formally specified algorithms, and is often solved by combinations of functions like fmap, foldr, and mapAccum, but mostly explicit recursion. Is there a more general and efficient, or just general and expressive, way to write code like this in Haskell?
There's a standard transformation that allows you to convert
∃x ∈ xs : P
to
exists (\x -> P) xs
If you need to produce a witness you can use find instead of exists.
The real nuisance of doing this kind of abstraction in Haskell as opposed to a logic language is that you really must pass the "universe" set xs as a parameter. I believe this is what brings in the "fuss" to which you refer in your title.
Of course you can, if you prefer, stuff the universal set (through which you are searching) into a monad. Then you can define your own versions of exists or find to work with the monadic state. To make it efficient, you can try Control.Monad.Logic, but it may involve breaking your head against Oleg's papers.
Anyway, the classic encoding is to replace all binding constructs, including existential and universal quantifiers, with lambdas, and proceed with appropriate function calls. My experience is that this encoding works even for complex nested queries with a lot of structure, but that it always feels clunky.
Maybe I don't understand something, but what's wrong with list comprehensions? Your second example becomes:
[(x,y,z) | x <- xs, y <- xs, z <- xs
, x /= y && y /= z && x /= z
, (p1 x y z) == (p2 x y z)]
This allows you to return values; to check if the formula is satisfied, just use null (it won't evaluate more than needed because of laziness).