(* Title: Subst/Unify
ID: $Id$
Author: Konrad Slind, Cambridge University Computer Laboratory
Copyright 1997 University of Cambridge
Unification algorithm
*)
Unify = Unifier + Recdef + Option +
consts
unifyRel :: "(('a uterm * 'a uterm) * ('a uterm * 'a uterm)) set"
unify :: "'a uterm * 'a uterm => ('a * 'a uterm) list option"
defs
(*Termination relation for the Unify function:
--either the set of variables decreases
--or the first argument does (in fact, both do)
*)
unifyRel_def "unifyRel == inv_image (finite_psubset <*lex*> measure uterm_size)
(%(M,N). (vars_of M Un vars_of N, M))"
recdef unify "unifyRel"
"unify(Const m, Const n) = (if (m=n) then Some[] else None)"
"unify(Const m, Comb M N) = None"
"unify(Const m, Var v) = Some[(v,Const m)]"
"unify(Var v, M) = (if (Var v <: M) then None else Some[(v,M)])"
"unify(Comb M N, Const x) = None"
"unify(Comb M N, Var v) = (if (Var v <: Comb M N) then None
else Some[(v,Comb M N)])"
"unify(Comb M1 N1, Comb M2 N2) =
(case unify(M1,M2)
of None => None
| Some theta => (case unify(N1 <| theta, N2 <| theta)
of None => None
| Some sigma => Some (theta <> sigma)))"
end