REWRITE_TAC : thm list -> tactic

SYNOPSIS

Rewrites a goal including built-in tautologies in the list of rewrites.

DESCRIPTION

Rewriting tactics in HOL provide a recursive left-to-right matching and rewriting facility that automatically decomposes subgoals and justifies segments of proof in which equational theorems are used, singly or collectively. These include the unfolding of definitions, and the substitution of equals for equals. Rewriting is used either to advance or to complete the decomposition of subgoals. REWRITE_TAC transforms (or solves) a goal by using as rewrite rules (i.e. as left-to-right replacement rules) the conclusions of the given list of (equational) theorems, as well as a set of built-in theorems (common tautologies) held in the ML variable basic_rewrites. Recognition of a tautology often terminates the subgoaling process (i.e. solves the goal). The equational rewrites generated are applied recursively and to arbitrary depth, with matching and instantiation of variables and type variables. A list of rewrites can set off an infinite rewriting process, and it is not, of course, decidable in general whether a rewrite set has that property. The order in which the rewrite theorems are applied is unspecified, and the user should not depend on any ordering. See GEN_REWRITE_TAC for more details on the rewriting process. Variants of REWRITE_TAC allow the use of a different set of rewrites. Some of them, such as PURE_REWRITE_TAC, exclude the basic tautologies from the possible transformations. ASM_REWRITE_TAC and others include the assumptions at the goal in the set of possible rewrites. Still other tactics allow greater control over the search for rewritable subterms. Several of them such as ONCE_REWRITE_TAC do not apply rewrites recursively. GEN_REWRITE_TAC allows a rewrite to be applied at a particular subterm.

FAILURE CONDITIONS

REWRITE_TAC does not fail. Certain sets of rewriting theorems on certain goals may cause a non-terminating sequence of rewrites. Divergent rewriting behaviour results from a term t being immediately or eventually rewritten to a term containing t as a sub-term. The exact behaviour depends on the HOL implementation; it may be possible (unfortunately) to fall into Lisp in this event.

EXAMPLE

The arithmetic theorem GT, |- !n m. m > n <=> n < m, is used below to advance a goal:

  # g `4 < 5`;;
  val it : goalstack = 1 subgoal (1 total)

  `4 < 5`

  # e(REWRITE_TAC[GT]);;
  val it : goalstack = 1 subgoal (1 total)

  `4 < 5`

It is used below with the theorem LT_0, |- !n. 0 < SUC n, to solve a goal:

  # g `SUC n > 0`;;
  Warning: Free variables in goal: n
  val it : goalstack = 1 subgoal (1 total)

  `SUC n > 0`

  # e(REWRITE_TAC[GT; LT_0]);;
  val it : goalstack = No subgoals

USES

Rewriting is a powerful and general mechanism in HOL, and an important part of many proofs. It relieves the user of the burden of directing and justifying a large number of minor proof steps. REWRITE_TAC fits a forward proof sequence smoothly into the general goal-oriented framework. That is, (within one subgoaling step) it produces and justifies certain forward inferences, none of which are necessarily on a direct path to the desired goal. REWRITE_TAC may be more powerful a tactic than is needed in certain situations; if efficiency is at stake, alternatives might be considered.