src/HOL/Induct/Comb.thy
 author paulson Wed, 07 May 1997 12:50:26 +0200 changeset 3120 c58423c20740 child 3309 992a25b24d0d permissions -rw-r--r--
New directory to contain examples of (co)inductive definitions
```
(*  Title:      HOL/ex/Comb.thy
ID:         \$Id\$
Author:     Lawrence C Paulson

Combinatory Logic example: the Church-Rosser Theorem
Curiously, combinators do not include free variables.

Example taken from
J. Camilleri and T. F. Melham.
Reasoning with Inductively Defined Relations in the HOL Theorem Prover.
Report 265, University of Cambridge Computer Laboratory, 1992.
*)

Comb = Trancl +

(** Datatype definition of combinators S and K, with infixed application **)
datatype comb = K
| S
| "#" comb comb (infixl 90)

(** Inductive definition of contractions, -1->
and (multi-step) reductions, --->
**)
consts
contract  :: "(comb*comb) set"
"-1->"    :: [comb,comb] => bool   (infixl 50)
"--->"    :: [comb,comb] => bool   (infixl 50)

translations
"x -1-> y" == "(x,y) : contract"
"x ---> y" == "(x,y) : contract^*"

inductive contract
intrs
K     "K#x#y -1-> x"
S     "S#x#y#z -1-> (x#z)#(y#z)"
Ap1   "x-1->y ==> x#z -1-> y#z"
Ap2   "x-1->y ==> z#x -1-> z#y"

(** Inductive definition of parallel contractions, =1=>
and (multi-step) parallel reductions, ===>
**)
consts
parcontract :: "(comb*comb) set"
"=1=>"    :: [comb,comb] => bool   (infixl 50)
"===>"    :: [comb,comb] => bool   (infixl 50)

translations
"x =1=> y" == "(x,y) : parcontract"
"x ===> y" == "(x,y) : parcontract^*"

inductive parcontract
intrs
refl  "x =1=> x"
K     "K#x#y =1=> x"
S     "S#x#y#z =1=> (x#z)#(y#z)"
Ap    "[| x=1=>y;  z=1=>w |] ==> x#z =1=> y#w"

(*Misc definitions*)
constdefs
I :: comb
"I == S#K#K"

(*confluence; Lambda/Commutation treats this more abstractly*)
diamond   :: "('a * 'a)set => bool"
"diamond(r) == ALL x y. (x,y):r -->
(ALL y'. (x,y'):r -->
(EX z. (y,z):r & (y',z) : r))"

end
```