src/HOL/Isar_Examples/Basic_Logic.thy

tuned document;

(*  Title:      HOL/Isar_Examples/Basic_Logic.thy
    Author:     Markus Wenzel, TU Muenchen

Basic propositional and quantifier reasoning.
*)

section \<open>Basic logical reasoning\<close>

theory Basic_Logic
imports Main
begin


subsection \<open>Pure backward reasoning\<close>

text \<open>In order to get a first idea of how Isabelle/Isar proof documents may
  look like, we consider the propositions \<open>I\<close>, \<open>K\<close>, and \<open>S\<close>. The following
  (rather explicit) proofs should require little extra explanations.\<close>

lemma I: "A \<longrightarrow> A"
proof
  assume A
  show A by fact
qed

lemma K: "A \<longrightarrow> B \<longrightarrow> A"
proof
  assume A
  show "B \<longrightarrow> A"
  proof
    show A by fact
  qed
qed

lemma S: "(A \<longrightarrow> B \<longrightarrow> C) \<longrightarrow> (A \<longrightarrow> B) \<longrightarrow> A \<longrightarrow> C"
proof
  assume "A \<longrightarrow> B \<longrightarrow> C"
  show "(A \<longrightarrow> B) \<longrightarrow> A \<longrightarrow> C"
  proof
    assume "A \<longrightarrow> B"
    show "A \<longrightarrow> C"
    proof
      assume A
      show C
      proof (rule mp)
        show "B \<longrightarrow> C" by (rule mp) fact+
        show B by (rule mp) fact+
      qed
    qed
  qed
qed

text \<open>Isar provides several ways to fine-tune the reasoning, avoiding
  excessive detail. Several abbreviated language elements are available,
  enabling the writer to express proofs in a more concise way, even without
  referring to any automated proof tools yet.

  First of all, proof by assumption may be abbreviated as a single dot.\<close>

lemma "A \<longrightarrow> A"
proof
  assume A
  show A by fact+
qed

text \<open>In fact, concluding any (sub-)proof already involves solving any
  remaining goals by assumption\footnote{This is not a completely trivial
  operation, as proof by assumption may involve full higher-order
  unification.}. Thus we may skip the rather vacuous body of the above proof
  as well.\<close>

lemma "A \<longrightarrow> A"
proof
qed

text \<open>Note that the \isacommand{proof} command refers to the \<open>rule\<close> method
  (without arguments) by default. Thus it implicitly applies a single rule,
  as determined from the syntactic form of the statements involved. The
  \isacommand{by} command abbreviates any proof with empty body, so the
  proof may be further pruned.\<close>

lemma "A \<longrightarrow> A"
  by rule

text \<open>Proof by a single rule may be abbreviated as double-dot.\<close>

lemma "A \<longrightarrow> A" ..

text \<open>Thus we have arrived at an adequate representation of the proof of a
  tautology that holds by a single standard rule.\footnote{Apparently, the
  rule here is implication introduction.}

  \<^medskip>
  Let us also reconsider \<open>K\<close>. Its statement is composed of iterated
  connectives. Basic decomposition is by a single rule at a time, which is
  why our first version above was by nesting two proofs.

  The \<open>intro\<close> proof method repeatedly decomposes a goal's
  conclusion.\footnote{The dual method is \<open>elim\<close>, acting on a goal's
  premises.}\<close>

lemma "A \<longrightarrow> B \<longrightarrow> A"
proof (intro impI)
  assume A
  show A by fact
qed

text \<open>Again, the body may be collapsed.\<close>

lemma "A \<longrightarrow> B \<longrightarrow> A"
  by (intro impI)

text \<open>Just like \<open>rule\<close>, the \<open>intro\<close> and \<open>elim\<close> proof methods pick standard
  structural rules, in case no explicit arguments are given. While implicit
  rules are usually just fine for single rule application, this may go too
  far with iteration. Thus in practice, \<open>intro\<close> and \<open>elim\<close> would be
  typically restricted to certain structures by giving a few rules only,
  e.g.\ \isacommand{proof}~\<open>(intro impI allI)\<close> to strip implications and
  universal quantifiers.

  Such well-tuned iterated decomposition of certain structures is the prime
  application of \<open>intro\<close> and \<open>elim\<close>. In contrast, terminal steps that solve
  a goal completely are usually performed by actual automated proof methods
  (such as \isacommand{by}~\<open>blast\<close>.\<close>


subsection \<open>Variations of backward vs.\ forward reasoning\<close>

text \<open>Certainly, any proof may be performed in backward-style only. On the
  other hand, small steps of reasoning are often more naturally expressed in
  forward-style. Isar supports both backward and forward reasoning as a
  first-class concept. In order to demonstrate the difference, we consider
  several proofs of \<open>A \<and> B \<longrightarrow> B \<and> A\<close>.

  The first version is purely backward.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof
  assume "A \<and> B"
  show "B \<and> A"
  proof
    show B by (rule conjunct2) fact
    show A by (rule conjunct1) fact
  qed
qed

text \<open>Above, the projection rules \<open>conjunct1\<close> / \<open>conjunct2\<close> had to be named
  explicitly, since the goals \<open>B\<close> and \<open>A\<close> did not provide any structural
  clue. This may be avoided using \isacommand{from} to focus on the \<open>A \<and> B\<close>
  assumption as the current facts, enabling the use of double-dot proofs.
  Note that \isacommand{from} already does forward-chaining, involving the
  \<open>conjE\<close> rule here.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof
  assume "A \<and> B"
  show "B \<and> A"
  proof
    from \<open>A \<and> B\<close> show B ..
    from \<open>A \<and> B\<close> show A ..
  qed
qed

text \<open>In the next version, we move the forward step one level upwards.
  Forward-chaining from the most recent facts is indicated by the
  \isacommand{then} command. Thus the proof of \<open>B \<and> A\<close> from \<open>A \<and> B\<close> actually
  becomes an elimination, rather than an introduction. The resulting proof
  structure directly corresponds to that of the \<open>conjE\<close> rule, including the
  repeated goal proposition that is abbreviated as \<open>?thesis\<close> below.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof
  assume "A \<and> B"
  then show "B \<and> A"
  proof                    -- \<open>rule \<open>conjE\<close> of \<open>A \<and> B\<close>\<close>
    assume B A
    then show ?thesis ..   -- \<open>rule \<open>conjI\<close> of \<open>B \<and> A\<close>\<close>
  qed
qed

text \<open>In the subsequent version we flatten the structure of the main body by
  doing forward reasoning all the time. Only the outermost decomposition
  step is left as backward.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof
  assume "A \<and> B"
  from \<open>A \<and> B\<close> have A ..
  from \<open>A \<and> B\<close> have B ..
  from \<open>B\<close> \<open>A\<close> show "B \<and> A" ..
qed

text \<open>We can still push forward-reasoning a bit further, even at the risk of
  getting ridiculous. Note that we force the initial proof step to do
  nothing here, by referring to the \<open>-\<close> proof method.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof -
  {
    assume "A \<and> B"
    from \<open>A \<and> B\<close> have A ..
    from \<open>A \<and> B\<close> have B ..
    from \<open>B\<close> \<open>A\<close> have "B \<and> A" ..
  }
  then show ?thesis ..         -- \<open>rule \<open>impI\<close>\<close>
qed

text \<open>
  \<^medskip>
  With these examples we have shifted through a whole range from purely
  backward to purely forward reasoning. Apparently, in the extreme ends we
  get slightly ill-structured proofs, which also require much explicit
  naming of either rules (backward) or local facts (forward).

  The general lesson learned here is that good proof style would achieve
  just the \<^emph>\<open>right\<close> balance of top-down backward decomposition, and
  bottom-up forward composition. In general, there is no single best way to
  arrange some pieces of formal reasoning, of course. Depending on the
  actual applications, the intended audience etc., rules (and methods) on
  the one hand vs.\ facts on the other hand have to be emphasized in an
  appropriate way. This requires the proof writer to develop good taste, and
  some practice, of course.

  \<^medskip>
  For our example the most appropriate way of reasoning is probably the
  middle one, with conjunction introduction done after elimination.\<close>

lemma "A \<and> B \<longrightarrow> B \<and> A"
proof
  assume "A \<and> B"
  then show "B \<and> A"
  proof
    assume B A
    then show ?thesis ..
  qed
qed



subsection \<open>A few examples from ``Introduction to Isabelle''\<close>

text \<open>We rephrase some of the basic reasoning examples of @{cite
  "isabelle-intro"}, using HOL rather than FOL.\<close>


subsubsection \<open>A propositional proof\<close>

text \<open>We consider the proposition \<open>P \<or> P \<longrightarrow> P\<close>. The proof below involves
  forward-chaining from \<open>P \<or> P\<close>, followed by an explicit case-analysis on
  the two \<^emph>\<open>identical\<close> cases.\<close>

lemma "P \<or> P \<longrightarrow> P"
proof
  assume "P \<or> P"
  then show P
  proof                    -- \<open>
    rule \<open>disjE\<close>: \smash{$\infer{C}{A \disj B & \infer*{C}{[A]} & \infer*{C}{[B]}}$}
\<close>
    assume P show P by fact
  next
    assume P show P by fact
  qed
qed

text \<open>Case splits are \<^emph>\<open>not\<close> hardwired into the Isar language as a
  special feature. The \isacommand{next} command used to separate the cases
  above is just a short form of managing block structure.

  \<^medskip>
  In general, applying proof methods may split up a goal into separate
  ``cases'', i.e.\ new subgoals with individual local assumptions. The
  corresponding proof text typically mimics this by establishing results in
  appropriate contexts, separated by blocks.

  In order to avoid too much explicit parentheses, the Isar system
  implicitly opens an additional block for any new goal, the
  \isacommand{next} statement then closes one block level, opening a new
  one. The resulting behaviour is what one would expect from separating
  cases, only that it is more flexible. E.g.\ an induction base case (which
  does not introduce local assumptions) would \<^emph>\<open>not\<close> require
  \isacommand{next} to separate the subsequent step case.

  \<^medskip>
  In our example the situation is even simpler, since the two cases actually
  coincide. Consequently the proof may be rephrased as follows.\<close>

lemma "P \<or> P \<longrightarrow> P"
proof
  assume "P \<or> P"
  then show P
  proof
    assume P
    show P by fact
    show P by fact
  qed
qed

text \<open>Again, the rather vacuous body of the proof may be collapsed.
  Thus the case analysis degenerates into two assumption steps, which
  are implicitly performed when concluding the single rule step of the
  double-dot proof as follows.\<close>

lemma "P \<or> P \<longrightarrow> P"
proof
  assume "P \<or> P"
  then show P ..
qed


subsubsection \<open>A quantifier proof\<close>

text \<open>To illustrate quantifier reasoning, let us prove
  \<open>(\<exists>x. P (f x)) \<longrightarrow> (\<exists>y. P y)\<close>. Informally, this holds because any \<open>a\<close> with
  \<open>P (f a)\<close> may be taken as a witness for the second existential statement.

  The first proof is rather verbose, exhibiting quite a lot of (redundant)
  detail. It gives explicit rules, even with some instantiation.
  Furthermore, we encounter two new language elements: the \isacommand{fix}
  command augments the context by some new ``arbitrary, but fixed'' element;
  the \isacommand{is} annotation binds term abbreviations by higher-order
  pattern matching.\<close>

lemma "(\<exists>x. P (f x)) \<longrightarrow> (\<exists>y. P y)"
proof
  assume "\<exists>x. P (f x)"
  then show "\<exists>y. P y"
  proof (rule exE)             --
    \<open>rule \<open>exE\<close>: \smash{$\infer{B}{\ex x A(x) & \infer*{B}{[A(x)]_x}}$}\<close>
    fix a
    assume "P (f a)" (is "P ?witness")
    then show ?thesis by (rule exI [of P ?witness])
  qed
qed

text \<open>While explicit rule instantiation may occasionally improve readability
  of certain aspects of reasoning, it is usually quite redundant. Above, the
  basic proof outline gives already enough structural clues for the system
  to infer both the rules and their instances (by higher-order unification).
  Thus we may as well prune the text as follows.\<close>

lemma "(\<exists>x. P (f x)) \<longrightarrow> (\<exists>y. P y)"
proof
  assume "\<exists>x. P (f x)"
  then show "\<exists>y. P y"
  proof
    fix a
    assume "P (f a)"
    then show ?thesis ..
  qed
qed

text \<open>Explicit \<open>\<exists>\<close>-elimination as seen above can become quite cumbersome in
  practice. The derived Isar language element ``\isakeyword{obtain}''
  provides a more handsome way to do generalized existence reasoning.\<close>

lemma "(\<exists>x. P (f x)) \<longrightarrow> (\<exists>y. P y)"
proof
  assume "\<exists>x. P (f x)"
  then obtain a where "P (f a)" ..
  then show "\<exists>y. P y" ..
qed

text \<open>Technically, \isakeyword{obtain} is similar to \isakeyword{fix} and
  \isakeyword{assume} together with a soundness proof of the elimination
  involved. Thus it behaves similar to any other forward proof element. Also
  note that due to the nature of general existence reasoning involved here,
  any result exported from the context of an \isakeyword{obtain} statement
  may \<^emph>\<open>not\<close> refer to the parameters introduced there.\<close>


subsubsection \<open>Deriving rules in Isabelle\<close>

text \<open>We derive the conjunction elimination rule from the corresponding
  projections. The proof is quite straight-forward, since Isabelle/Isar
  supports non-atomic goals and assumptions fully transparently.\<close>

theorem conjE: "A \<and> B \<Longrightarrow> (A \<Longrightarrow> B \<Longrightarrow> C) \<Longrightarrow> C"
proof -
  assume "A \<and> B"
  assume r: "A \<Longrightarrow> B \<Longrightarrow> C"
  show C
  proof (rule r)
    show A by (rule conjunct1) fact
    show B by (rule conjunct2) fact
  qed
qed

end