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\begin{isabelle}%
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%
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\begin{isamarkuptext}%
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Here is a simple example, the Fibonacci function:%
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\end{isamarkuptext}%
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\isacommand{consts}~fib~::~{"}nat~{\isasymRightarrow}~nat{"}\isanewline
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\isacommand{recdef}~fib~{"}measure({\isasymlambda}n.~n){"}\isanewline
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~~{"}fib~0~=~0{"}\isanewline
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~~{"}fib~1~=~1{"}\isanewline
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~~{"}fib~(Suc(Suc~x))~=~fib~x~+~fib~(Suc~x){"}%
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\begin{isamarkuptext}%
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\noindent
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The definition of \isa{fib} is accompanied by a \bfindex{measure function}
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\isa{\isasymlambda{}n.$\;$n} which maps the argument of \isa{fib} to a
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natural number. The requirement is that in each equation the measure of the
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argument on the left-hand side is strictly greater than the measure of the
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argument of each recursive call. In the case of \isa{fib} this is
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obviously true because the measure function is the identity and
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\isa{Suc(Suc~x)} is strictly greater than both \isa{x} and
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\isa{Suc~x}.
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Slightly more interesting is the insertion of a fixed element
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between any two elements of a list:%
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\end{isamarkuptext}%
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\isacommand{consts}~sep~::~{"}'a~*~'a~list~{\isasymRightarrow}~'a~list{"}\isanewline
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\isacommand{recdef}~sep~{"}measure~({\isasymlambda}(a,xs).~length~xs){"}\isanewline
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~~{"}sep(a,~[])~~~~~=~[]{"}\isanewline
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~~{"}sep(a,~[x])~~~~=~[x]{"}\isanewline
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~~{"}sep(a,~x\#y\#zs)~=~x~\#~a~\#~sep(a,y\#zs){"}%
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\begin{isamarkuptext}%
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\noindent
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This time the measure is the length of the list, which decreases with the
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recursive call; the first component of the argument tuple is irrelevant.
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Pattern matching need not be exhaustive:%
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\end{isamarkuptext}%
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\isacommand{consts}~last~::~{"}'a~list~{\isasymRightarrow}~'a{"}\isanewline
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\isacommand{recdef}~last~{"}measure~({\isasymlambda}xs.~length~xs){"}\isanewline
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~~{"}last~[x]~~~~~~=~x{"}\isanewline
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~~{"}last~(x\#y\#zs)~=~last~(y\#zs){"}%
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\begin{isamarkuptext}%
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Overlapping patterns are disambiguated by taking the order of equations into
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account, just as in functional programming:%
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\end{isamarkuptext}%
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\isacommand{consts}~sep1~::~{"}'a~*~'a~list~{\isasymRightarrow}~'a~list{"}\isanewline
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\isacommand{recdef}~sep1~{"}measure~({\isasymlambda}(a,xs).~length~xs){"}\isanewline
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~~{"}sep1(a,~x\#y\#zs)~=~x~\#~a~\#~sep1(a,y\#zs){"}\isanewline
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~~{"}sep1(a,~xs)~~~~~=~xs{"}%
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\begin{isamarkuptext}%
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\noindent
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This defines exactly the same function as \isa{sep} above, i.e.\
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\isa{sep1 = sep}.
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\begin{warn}
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\isacommand{recdef} only takes the first argument of a (curried)
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recursive function into account. This means both the termination measure
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and pattern matching can only use that first argument. In general, you will
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therefore have to combine several arguments into a tuple. In case only one
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argument is relevant for termination, you can also rearrange the order of
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arguments as in the following definition:
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\end{warn}%
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\end{isamarkuptext}%
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\isacommand{consts}~sep2~::~{"}'a~list~{\isasymRightarrow}~'a~{\isasymRightarrow}~'a~list{"}\isanewline
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\isacommand{recdef}~sep2~{"}measure~length{"}\isanewline
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~~{"}sep2~(x\#y\#zs)~=~({\isasymlambda}a.~x~\#~a~\#~sep2~zs~a){"}\isanewline
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~~{"}sep2~xs~~~~~~~=~({\isasymlambda}a.~xs){"}%
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\begin{isamarkuptext}%
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Because of its pattern-matching syntax, \isacommand{recdef} is also useful
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for the definition of non-recursive functions:%
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\end{isamarkuptext}%
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\isacommand{consts}~swap12~::~{"}'a~list~{\isasymRightarrow}~'a~list{"}\isanewline
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\isacommand{recdef}~swap12~{"}{\isabraceleft}{\isabraceright}{"}\isanewline
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~~{"}swap12~(x\#y\#zs)~=~y\#x\#zs{"}\isanewline
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~~{"}swap12~zs~~~~~~~=~zs{"}%
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\begin{isamarkuptext}%
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\noindent
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For non-recursive functions the termination measure degenerates to the empty
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set \isa{\{\}}.%
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\end{isamarkuptext}%
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\end{isabelle}%
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