src/HOL/Isar_Examples/Basic_Logic.thy

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

Basic propositional and quantifier reasoning.
*)

header {* Basic logical reasoning *}

theory Basic_Logic
imports Main
begin


subsection {* Pure backward reasoning *}

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

lemma I: "A --> A"
proof
  assume A
  show A by fact
qed

lemma K: "A --> B --> A"
proof
  assume A
  show "B --> A"
  proof
    show A by fact
  qed
qed

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

text {* 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. *}

lemma "A --> A"
proof
  assume A
  show A by fact+
qed

text {* 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. *}

lemma "A --> A"
proof
qed

text {* Note that the \isacommand{proof} command refers to the @{text
  rule} 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. *}

lemma "A --> A"
  by rule

text {* Proof by a single rule may be abbreviated as double-dot. *}

lemma "A --> A" ..

text {* 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.} *}

text {* Let us also reconsider @{text K}.  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 @{text intro} proof method repeatedly decomposes a goal's
  conclusion.\footnote{The dual method is @{text elim}, acting on a
  goal's premises.} *}

lemma "A --> B --> A"
proof (intro impI)
  assume A
  show A by fact
qed

text {* Again, the body may be collapsed. *}

lemma "A --> B --> A"
  by (intro impI)

text {* Just like @{text rule}, the @{text intro} and @{text elim}
  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, @{text intro} and @{text elim} would be typically
  restricted to certain structures by giving a few rules only, e.g.\
  \isacommand{proof}~@{text "(intro impI allI)"} to strip implications
  and universal quantifiers.

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


subsection {* Variations of backward vs.\ forward reasoning *}

text {* 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 @{text "A \<and> B \<longrightarrow> B \<and> A"}.

  The first version is purely backward. *}

lemma "A & B --> B & A"
proof
  assume "A & B"
  show "B & A"
  proof
    show B by (rule conjunct2) fact
    show A by (rule conjunct1) fact
  qed
qed

text {* Above, the @{text "conjunct_1/2"} projection rules had to be
  named explicitly, since the goals @{text B} and @{text A} did not
  provide any structural clue.  This may be avoided using
  \isacommand{from} to focus on the @{text "A \<and> B"} assumption as the
  current facts, enabling the use of double-dot proofs.  Note that
  \isacommand{from} already does forward-chaining, involving the
  @{text conjE} rule here. *}

lemma "A & B --> B & A"
proof
  assume "A & B"
  show "B & A"
  proof
    from `A & B` show B ..
    from `A & B` show A ..
  qed
qed

text {* 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 @{text "B \<and> A"}
  from @{text "A \<and> B"} actually becomes an elimination, rather than an
  introduction.  The resulting proof structure directly corresponds to
  that of the @{text conjE} rule, including the repeated goal
  proposition that is abbreviated as @{text ?thesis} below. *}

lemma "A & B --> B & A"
proof
  assume "A & B"
  then show "B & A"
  proof                    -- {* rule @{text conjE} of @{text "A \<and> B"} *}
    assume B A
    then show ?thesis ..   -- {* rule @{text conjI} of @{text "B \<and> A"} *}
  qed
qed

text {* 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. *}

lemma "A & B --> B & A"
proof
  assume "A & B"
  from `A & B` have A ..
  from `A & B` have B ..
  from `B` `A` show "B & A" ..
qed

text {* 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 ``-'' proof method. *}

lemma "A & B --> B & A"
proof -
  {
    assume "A & B"
    from `A & B` have A ..
    from `A & B` have B ..
    from `B` `A` have "B & A" ..
  }
  then show ?thesis ..         -- {* rule @{text impI} *}
qed

text {* \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{right} 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. *}

text {* For our example the most appropriate way of reasoning is
  probably the middle one, with conjunction introduction done after
  elimination. *}

lemma "A & B --> B & A"
proof
  assume "A & B"
  then show "B & A"
  proof
    assume B A
    then show ?thesis ..
  qed
qed



subsection {* A few examples from ``Introduction to Isabelle'' *}

text {* We rephrase some of the basic reasoning examples of
  \cite{isabelle-intro}, using HOL rather than FOL. *}


subsubsection {* A propositional proof *}

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

lemma "P | P --> P"
proof
  assume "P | P"
  then show P
  proof                    -- {*
    rule @{text disjE}: \smash{$\infer{C}{A \disj B & \infer*{C}{[A]} & \infer*{C}{[B]}}$}
  *}
    assume P show P by fact
  next
    assume P show P by fact
  qed
qed

text {* Case splits are \emph{not} 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 behavior 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{not} 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. *}

lemma "P | P --> P"
proof
  assume "P | P"
  then show P
  proof
    assume P
    show P by fact
    show P by fact
  qed
qed

text {* 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. *}

lemma "P | P --> P"
proof
  assume "P | P"
  then show P ..
qed


subsubsection {* A quantifier proof *}

text {* To illustrate quantifier reasoning, let us prove @{text
  "(\<exists>x. P (f x)) \<longrightarrow> (\<exists>y. P y)"}.  Informally, this holds because any
  @{text a} with @{text "P (f a)"} 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. *}

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

text {* 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. *}

lemma "(EX x. P (f x)) --> (EX y. P y)"
proof
  assume "EX x. P (f x)"
  then show "EX y. P y"
  proof
    fix a
    assume "P (f a)"
    then show ?thesis ..
  qed
qed

text {* Explicit @{text \<exists>}-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. *}

lemma "(EX x. P (f x)) --> (EX y. P y)"
proof
  assume "EX x. P (f x)"
  then obtain a where "P (f a)" ..
  then show "EX y. P y" ..
qed

text {* 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{not} refer
  to the parameters introduced there. *}


subsubsection {* Deriving rules in Isabelle *}

text {* 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. *}

theorem conjE: "A & B ==> (A ==> B ==> C) ==> C"
proof -
  assume "A & B"
  assume r: "A ==> B ==> C"
  show C
  proof (rule r)
    show A by (rule conjunct1) fact
    show B by (rule conjunct2) fact
  qed
qed

end