commit by user andrea

diff --git a/weblib/tutorial/chapter3.ma b/weblib/tutorial/chapter3.ma
index 849b067e7f587d49175a3fad3abffbaf8ec6a493..a75b4ce1e023e8e09bf20662f4718ee2e7ae0e80 100644 (file)
--- a/weblib/tutorial/chapter3.ma
+++ b/weblib/tutorial/chapter3.ma
@@ -5,7 +5,7 @@ include "basics/bool.ma".
 (* Matita supports polymorphic data types. The most typical case are polymorphic
 lists, parametric in the type of their elements: *)
 
-inductive list (A:Type[0]) : Type[0] :=
+inductive list (A:Type[0]) : Type[0] ≝
   | nil: list A
   | cons: A -> list A -> list A.
 
@@ -77,9 +77,10 @@ proceed by induction on l *)
 lemma append_nil: ∀A.∀l:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A.l \ 5a title="append" href="cic:/fakeuri.def(1)"\ 6@\ 5/a\ 6 \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6] \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 l.
 #A #l (elim l) normalize // qed.
 
-(* similarly, we can define the two functions head and tail. We should decide what to do in 
-case the input list is empty. For tl, it is natural to return the empty list; for hd, we take 
-in input a default element d of type A to return in this case. *)
+(* similarly, we can define the two functions head and tail. Since we can only define
+total functions, we should decide what to do in case the input list is empty. For tl, it 
+is natural to return the empty list; for hd, we take in input a default element d of type 
+A to return in this case. *)
 
 definition head ≝ λA.λl: \ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A.λd:A.
   match l with [ nil ⇒ d | cons a _ ⇒ a].
@@ -122,7 +123,10 @@ normalize // qed.
 example ex_nth: \ 5a href="cic:/matita/tutorial/chapter3/nth.fix(0,0,2)"\ 6nth\ 5/a\ 6 (\ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,2,0)"\ 6S\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,1,0)"\ 6O\ 5/a\ 6) ? (\ 5a href="cic:/matita/tutorial/chapter3/list.con(0,2,1)"\ 6cons\ 5/a\ 6 ? (\ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,2,0)"\ 6S\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,1,0)"\ 6O\ 5/a\ 6) (\ 5a href="cic:/matita/tutorial/chapter3/list.con(0,2,1)"\ 6cons\ 5/a\ 6 ? \ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,1,0)"\ 6O\ 5/a\ 6 \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6])) \ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,1,0)"\ 6O\ 5/a\ 6 \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter2/nat.con(0,1,0)"\ 6O\ 5/a\ 6.
 normalize // qed.
 
-lemma  length_add: ∀A.∀l1,l2:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A. 
+(* Proving that the length of l1@l2 is the sum of the lengths of l1
+and l2 just requires a trivial induction on the first list. *)
+
+ lemma  length_add: ∀A.∀l1,l2:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A. 
   \ 5a href="cic:/matita/tutorial/chapter3/length.fix(0,1,1)"\ 6length\ 5/a\ 6 ? (l1\ 5a title="append" href="cic:/fakeuri.def(1)"\ 6@\ 5/a\ 6l2) \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter2/add.fix(0,0,1)"\ 6add\ 5/a\ 6 (\ 5a href="cic:/matita/tutorial/chapter3/length.fix(0,1,1)"\ 6length\ 5/a\ 6 ? l1) (\ 5a href="cic:/matita/tutorial/chapter3/length.fix(0,1,1)"\ 6length\ 5/a\ 6 ? l2).
 #A #l1 elim l1 normalize // qed. 
 
@@ -164,18 +168,26 @@ lemma nil_to_nil:  ∀A.∀l1,l2:\ 5a href="cic:/matita/tutorial/chapter3/list.ind
   l1\ 5a title="append" href="cic:/fakeuri.def(1)"\ 6@\ 5/a\ 6l2 \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6] → l1 \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6] \ 5a title="logical and" href="cic:/fakeuri.def(1)"\ 6∧\ 5/a\ 6 l2 \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6].
 #A #l1 cases l1 normalize /2/ #a #tl #l2 #H @\ 5a href="cic:/matita/basics/logic/False_ind.fix(0,1,1)"\ 6False_ind\ 5/a\ 6 /2/ qed. 
 
-(* iterators *)
+(* Let us come to some important, higher order, polymorphic functionals 
+acting over lists. A typical example is the map function, taking a function
+f:A → B, a list l = [a1; a2; ... ; an] and returning the list 
+[f a1; f a2; ... ; f an]. *)
 
 let rec map (A,B:Type[0]) (f: A → B) (l:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A) on l: \ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 B ≝
  match l with [ nil ⇒ \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6] | cons x tl ⇒ f x \ 5a title="cons" href="cic:/fakeuri.def(1)"\ 6:\ 5/a\ 6: (map A B f tl)].
-  
-let rec foldr (A,B:Type[0]) (f:A → B → B) (b:B) (l:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A) on l :B ≝  
- match l with [ nil ⇒ b | cons a l ⇒ f a (foldr A B f b l)].
- 
+
+(* Another simple example is the filter function that given a list l: list A and
+a boolean test p:A → bool returns the sublist of elements satisfying the test. *)
+
 definition filter ≝ 
   λT.λp:T → \ 5a href="cic:/matita/basics/bool/bool.ind(1,0,0)"\ 6bool\ 5/a\ 6.
   \ 5a href="cic:/matita/tutorial/chapter3/foldr.fix(0,4,1)"\ 6foldr\ 5/a\ 6 T (\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 T) (λx,l0.\ 5a href="cic:/matita/basics/bool/if_then_else.def(1)"\ 6if_then_else\ 5/a\ 6 ? (p x) (x\ 5a title="cons" href="cic:/fakeuri.def(1)"\ 6:\ 5/a\ 6:l0) l0) \ 5a title="nil" href="cic:/fakeuri.def(1)"\ 6[\ 5/a\ 6].
 
+(* Here are a couple of simple lemmas on the behaviour of the filter function. 
+It is often convenient to state such lemmas, in order to be able to use rewriting 
+as an alternative to reduction in proofs: reduction is a bit difficult to control.
+*)
+
 lemma filter_true : ∀A,l,a,p. p a \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a href="cic:/matita/basics/bool/bool.con(0,1,0)"\ 6true\ 5/a\ 6 → 
   \ 5a href="cic:/matita/tutorial/chapter3/filter.def(2)"\ 6filter\ 5/a\ 6 A p (a\ 5a title="cons" href="cic:/fakeuri.def(1)"\ 6:\ 5/a\ 6:l) \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 a \ 5a title="cons" href="cic:/fakeuri.def(1)"\ 6:\ 5/a\ 6: \ 5a href="cic:/matita/tutorial/chapter3/filter.def(2)"\ 6filter\ 5/a\ 6 A p l.
 #A #l #a #p #pa (elim l) normalize >pa // qed.
@@ -184,6 +196,13 @@ lemma filter_false : ∀A,l,a,p. p a \ 5a title="leibnitz's equality" href="cic:/f
   \ 5a href="cic:/matita/tutorial/chapter3/filter.def(2)"\ 6filter\ 5/a\ 6 A p (a\ 5a title="cons" href="cic:/fakeuri.def(1)"\ 6:\ 5/a\ 6:l) \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter3/filter.def(2)"\ 6filter\ 5/a\ 6 A p l.
 #A #l #a #p #pa (elim l) normalize >pa normalize // qed.
 
+(* The most typical list iterator is the fold function, that taken
+a list l = [a1; a2; ... an], a base value b, and a function f: A → B → B 
+returns f a1 (f a2 (... (f an b)...)). *)
+
+let rec foldr (A,B:Type[0]) (f:A → B → B) (b:B) (l:\ 5a href="cic:/matita/tutorial/chapter3/list.ind(1,0,1)"\ 6list\ 5/a\ 6 A) on l :B ≝  
+  match l with [ nil ⇒ b | cons a l ⇒ f a (foldr A B f b l)].
+
 theorem eq_map : ∀A,B,f,g,l. (∀x.f x \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 g x) → \ 5a href="cic:/matita/tutorial/chapter3/map.fix(0,3,1)"\ 6map\ 5/a\ 6 \ 5span style="text-decoration: underline;"\ 6\ 5/span\ 6A B f l \ 5a title="leibnitz's equality" href="cic:/fakeuri.def(1)"\ 6=\ 5/a\ 6 \ 5a href="cic:/matita/tutorial/chapter3/map.fix(0,3,1)"\ 6map\ 5/a\ 6 A B g l.
 #A #B #f #g #l #eqfg (elim l) normalize // qed.
 
@@ -246,4 +265,5 @@ theorem fold_sum: ∀A,B. ∀I,J:\ 5a href="cic:/matita/tutorial/chapter3/list.ind
     \ 5a title="\fold" href="cic:/fakeuri.def(1)"\ 6\fold\ 5/a\ 6[op,nil]_{i∈(I\ 5a title="append" href="cic:/fakeuri.def(1)"\ 6@\ 5/a\ 6J)} (f i).
 #A #B #I #J #nil #op #f (elim I) normalize 
   [>\ 5a href="cic:/matita/tutorial/chapter3/nill.fix(0,2,2)"\ 6nill\ 5/a\ 6 //|#a #tl #Hind <\ 5a href="cic:/matita/tutorial/chapter3/assoc.fix(0,2,2)"\ 6assoc\ 5/a\ 6 //]
-qed.
\ No newline at end of file
+qed.
+