doc-src/TutorialI/Datatype/Fundata.thy

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theory Fundata = Main:
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datatype ('a,'i)bigtree = Tip | Branch 'a "'i \\<Rightarrow> ('a,'i)bigtree"

text{*\noindent
Parameter @{typ"'a"} is the type of values stored in
the @{term"Branch"}es of the tree, whereas @{typ"'i"} is the index
type over which the tree branches. If @{typ"'i"} is instantiated to
@{typ"bool"}, the result is a binary tree; if it is instantiated to
@{typ"nat"}, we have an infinitely branching tree because each node
has as many subtrees as there are natural numbers. How can we possibly
write down such a tree? Using functional notation! For example, the term
\begin{quote}
@{term[display]"Branch 0 (%i. Branch i (%n. Tip))"}
\end{quote}
of type @{typ"(nat,nat)bigtree"} is the tree whose
root is labeled with 0 and whose $i$th subtree is labeled with $i$ and
has merely @{term"Tip"}s as further subtrees.

Function @{term"map_bt"} applies a function to all labels in a @{text"bigtree"}:
*}

consts map_bt :: "('a \\<Rightarrow> 'b) \\<Rightarrow> ('a,'i)bigtree \\<Rightarrow> ('b,'i)bigtree"
primrec
"map_bt f Tip          = Tip"
"map_bt f (Branch a F) = Branch (f a) (\\<lambda>i. map_bt f (F i))"

text{*\noindent This is a valid \isacommand{primrec} definition because the
recursive calls of @{term"map_bt"} involve only subtrees obtained from
@{term"F"}, i.e.\ the left-hand side. Thus termination is assured.  The
seasoned functional programmer might have written @{term"map_bt f o F"}
instead of @{term"%i. map_bt f (F i)"}, but the former is not accepted by
Isabelle because the termination proof is not as obvious since
@{term"map_bt"} is only partially applied.

The following lemma has a canonical proof  *}

lemma "map_bt (g o f) T = map_bt g (map_bt f T)";
by(induct_tac "T", simp_all)

text{*\noindent
but it is worth taking a look at the proof state after the induction step
to understand what the presence of the function type entails:
\begin{isabelle}
~1.~map\_bt~g~(map\_bt~f~Tip)~=~map\_bt~(g~{\isasymcirc}~f)~Tip\isanewline
~2.~{\isasymAnd}a~F.\isanewline
~~~~~~{\isasymforall}x.~map\_bt~g~(map\_bt~f~(F~x))~=~map\_bt~(g~{\isasymcirc}~f)~(F~x)~{\isasymLongrightarrow}\isanewline
~~~~~~map\_bt~g~(map\_bt~f~(Branch~a~F))~=~map\_bt~(g~{\isasymcirc}~f)~(Branch~a~F)%
\end{isabelle}
*}
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end
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