pureSimps.pure

STRUCTURE

SYNOPSIS

A simpset containing only the conditional rewrite generator and no additional rewrites.

LIBRARY

pureSimps

DESCRIPTION

This simpset sits at the root of the simpset hierarchy. It contains no rewrites, congruences, conversions or decision procedures. Instead it contains just the code which converts theorems passed to it as context into (possibly conditional) rewrites.

Simplification with pure_ss is analogous to rewriting with PURE_REWRITE_TAC and others. The only difference is that the theorems passed to SIMP_TAC pure_ss are interpreted as conditional rewrite rules. Though the pure_ss can’t take advantage of extra contextual information garnered through congruences, it can still discharge side conditions. (This is demonstrated in the examples below.)

FAILURE

Can’t fail, as it is not a functional value.

EXAMPLE

The theorem ADD_EQ_SUB from arithmeticTheory states that

   |- !m n p. n <= p ==> ((m + n = p) = m = p - n)

We can use this result to make progress with the following goal in conjunction with pure_ss in a way that no form of REWRITE_TAC could:

   - ASM_SIMP_TAC pure_ss [ADD_EQ_SUB] ([--`x <= y`--], --`z + x = y`--);
   > val it = ([([`x <= y`], `z = y - x`)], fn) : tactic_result

This example illustrates the way in which the simplifier can do conditional rewriting. However, the lack of the congruence for implications means that using pure_ss will not be able to discharge the side condition in the goal below:

   - SIMP_TAC pure_ss [ADD_EQ_SUB] ([], --`x <= y ==> (z + x = y)`--);
   > val it = ([([], `x <= y ==> (z + x = y)`)], fn) : tactic_result

As bool_ss has the relevant congruence included, it does make progress in the same situation:

   - SIMP_TAC bool_ss [ADD_EQ_SUB] ([], --`x <= y ==> (z + x = y)`--);
   > val it = ([([], `x <= y ==> (z = y - x)`)], fn) : tactic_result

USES

The pure_ss simpset might be used in the most delicate simplification situations, or, mimicking the way it is used within the distribution itself, as a basis for the construction of other simpsets.

COMMENTS

There is also a pureSimps.PURE_ss ssfrag value. Its usefulness is questionable.

SEEALSO

bool_ss, PURE_REWRITE_TAC, SIMP_CONV, SIMP_TAC