src/HOL/Isar_examples/BasicLogic.thy

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(*  Title:      HOL/Isar_examples/BasicLogic.thy
    ID:         $Id$
    Author:     Markus Wenzel, TU Muenchen

Basic propositional and quantifier reasoning.
*)

header {* Basic logical reasoning *}

theory BasicLogic 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 \name{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 \name{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 \name{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