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问题:
I have the following Isabelle goal:
lemma "⟦ if foo then a ≠ a else b ≠ b ⟧ ⟹ False"
None of the tactics simp
, fast
, clarsimp
, blast
, fastforce
, etc. make any progress on the goal, despite it being quite simple.
Why doesn't Isabelle just simplify the body of the if
construct so that both "a ≠ a" and "b ≠ b" become False
, and hence solve the goal?
回答1:
The if_weak_cong
congruence rule
By default, Isabelle includes a set of "congruence rules" that affect where simplification takes place. In particular, a default congruence rule is if_weak_cong
, as follows:
b = c ⟹ (if b then x else y) = (if c then x else y)
This congruence rule tells the simplifier to simplify the condition of the if
statement (the b = c
) but never attempt to simplify the body of the if
statement.
You can either disable the congruence rule using:
apply (simp cong del: if_weak_cong)
or override it with an alternative (more powerful) congruence rule:
apply (simp cong: if_cong)
Both of these will solve the above lemma.
Why if_weak_cong
is in the default cong set
Another reasonable question might be: "Why would if_weak_cong
be in the default congruence set if it causes problems like the above?"
One motivation is to prevent the simplifier from unfolding a recursive function infinitely, such as in the following case:
fun fact where
"fact (n :: nat) = (if n = 0 then 1 else (n * fact (n - 1)))"
in this case,
lemma "fact 3 = 6"
by simp
solves the goal, while
lemma "fact 3 = 6"
by (simp cong del: if_weak_cong)
sends the simplifier into a loop, because the right-hand-side of the fact
definition continually unfolds.
This second scenario tends to occur more frequently than the scenario in the original question, which motivates if_weak_cong
being the default.
回答2:
Split rules
Besides case analysis and congruence rules, there is a third way to solve this goal with the simplifier: The splitter. The splitter allows the simplifier to perform a limited form of case analysis on its own. It is only run when the term cannot be simplified any further on its own (splitting cases can easily lead to an explosion of the goal).
The lemma split_if_asm
instructs the splitter to split an if
in the assumptions:
lemma "⟦ if foo then a ≠ a else b ≠ b ⟧ ⟹ False"
by (simp split: split_if_asm)
A single-step split can be performed with the split
method:
lemma "⟦ if foo then a ≠ a else b ≠ b ⟧ ⟹ False"
apply (split split_if_asm)
apply simp_all
done
Note that the rule to split an if
in the conclusion (split_if
) is part of the default setup.
BTW, for each datatype t
, the datatype package provides the split rules t.split
and t.split_asm
which provide case analysis for case
expressions involving the type t
.
回答3:
Congruence rules
As already noted in the other answers, the if_weak_cong
congruence rule prevents the simplifier from simplifying the branches of the if statement. In this answer, I want to elaborate a bit on the use of congruence rules by the simplifier.
For further information, see also the chapter about the Simplifier in the Isabelle/Isar Reference Manual (in particular section 9.3.2).
Congruence rules control how the simplifier descends into terms. They can be used to limit rewriting and to provide additional assumptions. By default, if the simplifier encounters a function application s t
it will descend both into s
and t
to rewrite them to s'
and t'
, before trying to rewrite the resulting term s' t'
.
For each constant (or variable) c one can register a single congruence rule. The rule if_weak_cong
is registered by default the constant If
(which is underlying the if ... then ... else ...
syntax):
?b = ?c ⟹ (if ?b then ?x else ?y) = (if ?c then ?x else ?y)
This reads as: "If you encounter a term if ?b then ?x else ?y
and ?b
can be simplified to ?c
, then rewrite if ?b then ?x else ?y
to if ?c then ?x else ?y
". As congruence rules replace the default strategy, this forbids any rewriting of ?x
and ?y
.
An alternative to if_weak_cong
is the strong congruence rule if_cong
:
⟦ ?b = ?c; (?c ⟹ ?x = ?u); (¬ ?c ⟹ ?y = ?v) ⟧
⟹ (if ?b then ?x else ?y) = (if ?c then ?u else ?v)
Note the two assumptions (?c ⟹ ?x = ?u)
and (¬ ?c ⟹ ?y = ?v)
: They tell the simplifier that it may assume that the condition holds (or holds not) when simplifying the left (or right) branch of the if.
As an example, consider the behaviour of the simplifier on the goal
if foo ∨ False then ¬foo ∨ False else foo ⟹ False
and assume that we know nothing about foo
. Then,
apply simp
: with the rule if_weak_cong
, this will be simplified to
if foo then ¬ foo ∨ False else foo ⟹ False
, only the condition is rewritten
apply (simp cong del: if_weak_cong)
: Without any congruence rule, this will be
simplified to
if foo then ¬ foo else foo ⟹ False
, as the condition and the branches are rewritten
apply (simp cong: if_cong del: if_cancel)
: With the rule if_cong
, this goal will
simplified to
if foo then False else False ⟹ False
: The condition foo ∨ False
will be
rewritten to foo
. For the two branches, the simplifier now rewrites
foo ⟹ ¬foo ∨ False
and ¬foo ⟹ foo ∨ False
, both of which obviously rewrite to
False.
(I removed if_cancel
, which would usually solve the remaining goal completely)
回答4:
Another natural way of making progress in a proof containing if _ then _ else _
is case analysis on the condition, e.g.,
lemma "(if foo then a ~= a else b ~= b) ==> False"
by (cases foo) simp_all
or if foo
is not a free variable, but bound by an outermost meta-level universal quantifier (as is often the case in apply-scripts):
lemma "!!foo. (if foo then a ~= a else b ~= b) ==> False"
apply (case_tac foo)
apply simp_all
done
Unfortunately, if foo
is bound by another kind of quantifier, e.g.
lemma "ALL foo. (if foo then a ~= a else b ~= b) ==> False"
or by a meta-level universal quantifier in a nested assumption, e.g.
lemma "True ==> (!!foo. (if foo then a ~= a else b ~= b)) ==> False"
neither cases
nor case_tac
are applicable.
Note: See also here for the (slight) difference between cases
and case_tac
.