3 \def\isabellecontext{Partial}%
7 Throughout the tutorial we have emphasized the fact that all functions
8 in HOL are total. Hence we cannot hope to define truly partial
9 functions. The best we can do are functions that are
10 \emph{underdefined}\index{underdefined function}:
11 for certain arguments we only know that a result
12 exists, but we don't know what it is. When defining functions that are
13 normally considered partial, underdefinedness turns out to be a very
14 reasonable alternative.
16 We have already seen an instance of underdefinedness by means of
17 non-exhaustive pattern matching: the definition of \isa{last} in
18 \S\ref{sec:recdef-examples}. The same is allowed for \isacommand{primrec}%
20 \isacommand{consts}\ hd\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}{\isacharprime}a\ list\ {\isasymRightarrow}\ {\isacharprime}a{\isachardoublequote}\isanewline
21 \isacommand{primrec}\ {\isachardoublequote}hd\ {\isacharparenleft}x{\isacharhash}xs{\isacharparenright}\ {\isacharequal}\ x{\isachardoublequote}%
22 \begin{isamarkuptext}%
24 although it generates a warning.
26 Even ordinary definitions allow underdefinedness, this time by means of
29 \isacommand{constdefs}\ minus\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}nat\ {\isasymRightarrow}\ nat\ {\isasymRightarrow}\ nat{\isachardoublequote}\isanewline
30 {\isachardoublequote}n\ {\isasymle}\ m\ {\isasymLongrightarrow}\ minus\ m\ n\ {\isasymequiv}\ m\ {\isacharminus}\ n{\isachardoublequote}%
31 \begin{isamarkuptext}%
32 The rest of this section is devoted to the question of how to define
33 partial recursive functions by other means that non-exhaustive pattern
37 \isamarkupsubsubsection{Guarded recursion%
40 \begin{isamarkuptext}%
41 Neither \isacommand{primrec} nor \isacommand{recdef} allow to
42 prefix an equation with a condition in the way ordinary definitions do
43 (see \isa{minus} above). Instead we have to move the condition over
44 to the right-hand side of the equation. Given a partial function $f$
45 that should satisfy the recursion equation $f(x) = t$ over its domain
46 $dom(f)$, we turn this into the \isacommand{recdef}
48 \ \ \ \ \ f\ x\ {\isacharequal}\ {\isacharparenleft}if\ x\ {\isasymin}\ dom\ f\ then\ t\ else\ arbitrary{\isacharparenright}%
50 where \isa{arbitrary} is a predeclared constant of type \isa{{\isacharprime}a}
51 which has no definition. Thus we know nothing about its value,
52 which is ideal for specifying underdefined functions on top of it.
54 As a simple example we define division on \isa{nat}:%
56 \isacommand{consts}\ divi\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}nat\ {\isasymtimes}\ nat\ {\isasymRightarrow}\ nat{\isachardoublequote}\isanewline
57 \isacommand{recdef}\ divi\ {\isachardoublequote}measure{\isacharparenleft}{\isasymlambda}{\isacharparenleft}m{\isacharcomma}n{\isacharparenright}{\isachardot}\ m{\isacharparenright}{\isachardoublequote}\isanewline
58 \ \ {\isachardoublequote}divi{\isacharparenleft}m{\isacharcomma}n{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ n\ {\isacharequal}\ {\isadigit{0}}\ then\ arbitrary\ else\isanewline
59 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ if\ m\ {\isacharless}\ n\ then\ {\isadigit{0}}\ else\ divi{\isacharparenleft}m{\isacharminus}n{\isacharcomma}n{\isacharparenright}{\isacharplus}{\isadigit{1}}{\isacharparenright}{\isachardoublequote}%
60 \begin{isamarkuptext}%
61 \noindent Of course we could also have defined
62 \isa{divi\ {\isacharparenleft}m{\isacharcomma}\ {\isadigit{0}}{\isacharparenright}} to be some specific number, for example 0. The
63 latter option is chosen for the predefined \isa{div} function, which
64 simplifies proofs at the expense of moving further away from the
65 standard mathematical divison function.
67 As a more substantial example we consider the problem of searching a graph.
68 For simplicity our graph is given by a function (\isa{f}) of
69 type \isa{{\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a} which
70 maps each node to its successor, and the task is to find the end of a chain,
71 i.e.\ a node pointing to itself. Here is a first attempt:
73 \ \ \ \ \ find\ {\isacharparenleft}f{\isacharcomma}\ x{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ f\ x\ {\isacharequal}\ x\ then\ x\ else\ find\ {\isacharparenleft}f{\isacharcomma}\ f\ x{\isacharparenright}{\isacharparenright}%
75 This may be viewed as a fixed point finder or as one half of the well known
76 \emph{Union-Find} algorithm.
77 The snag is that it may not terminate if \isa{f} has nontrivial cycles.
78 Phrased differently, the relation%
80 \isacommand{constdefs}\ step{\isadigit{1}}\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}{\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isacharparenright}\ {\isasymRightarrow}\ {\isacharparenleft}{\isacharprime}a\ {\isasymtimes}\ {\isacharprime}a{\isacharparenright}set{\isachardoublequote}\isanewline
81 \ \ {\isachardoublequote}step{\isadigit{1}}\ f\ {\isasymequiv}\ {\isacharbraceleft}{\isacharparenleft}y{\isacharcomma}x{\isacharparenright}{\isachardot}\ y\ {\isacharequal}\ f\ x\ {\isasymand}\ y\ {\isasymnoteq}\ x{\isacharbraceright}{\isachardoublequote}%
82 \begin{isamarkuptext}%
84 must be well-founded. Thus we make the following definition:%
86 \isacommand{consts}\ find\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}{\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isacharparenright}\ {\isasymtimes}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isachardoublequote}\isanewline
87 \isacommand{recdef}\ find\ {\isachardoublequote}same{\isacharunderscore}fst\ {\isacharparenleft}{\isasymlambda}f{\isachardot}\ wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}{\isacharparenright}\ step{\isadigit{1}}{\isachardoublequote}\isanewline
88 \ \ {\isachardoublequote}find{\isacharparenleft}f{\isacharcomma}x{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}\isanewline
89 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ then\ if\ f\ x\ {\isacharequal}\ x\ then\ x\ else\ find{\isacharparenleft}f{\isacharcomma}\ f\ x{\isacharparenright}\isanewline
90 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ else\ arbitrary{\isacharparenright}{\isachardoublequote}\isanewline
91 {\isacharparenleft}\isakeyword{hints}\ recdef{\isacharunderscore}simp{\isacharcolon}same{\isacharunderscore}fst{\isacharunderscore}def\ step{\isadigit{1}}{\isacharunderscore}def{\isacharparenright}%
92 \begin{isamarkuptext}%
94 The recursion equation itself should be clear enough: it is our aborted
95 first attempt augmented with a check that there are no non-trivial loops.
97 What complicates the termination proof is that the argument of
98 \isa{find} is a pair. To express the required well-founded relation
99 we employ the predefined combinator \isa{same{\isacharunderscore}fst} of type
101 \ \ \ \ \ {\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ bool{\isacharparenright}\ {\isasymRightarrow}\ {\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ {\isacharparenleft}{\isacharprime}b{\isasymtimes}{\isacharprime}b{\isacharparenright}set{\isacharparenright}\ {\isasymRightarrow}\ {\isacharparenleft}{\isacharparenleft}{\isacharprime}a{\isasymtimes}{\isacharprime}b{\isacharparenright}\ {\isasymtimes}\ {\isacharparenleft}{\isacharprime}a{\isasymtimes}{\isacharprime}b{\isacharparenright}{\isacharparenright}set%
105 \ \ \ \ \ same{\isacharunderscore}fst\ P\ R\ {\isasymequiv}\ {\isacharbraceleft}{\isacharparenleft}{\isacharparenleft}x{\isacharprime}{\isacharcomma}\ y{\isacharprime}{\isacharparenright}{\isacharcomma}\ x{\isacharcomma}\ y{\isacharparenright}{\isachardot}\ x{\isacharprime}\ {\isacharequal}\ x\ {\isasymand}\ P\ x\ {\isasymand}\ {\isacharparenleft}y{\isacharprime}{\isacharcomma}\ y{\isacharparenright}\ {\isasymin}\ R\ x{\isacharbraceright}%
107 This combinator is designed for recursive functions on pairs where the first
108 component of the argument is passed unchanged to all recursive
109 calls. Given a constraint on the first component and a relation on the second
110 component, \isa{same{\isacharunderscore}fst} builds the required relation on pairs.
111 The theorem \begin{isabelle}%
112 \ \ \ \ \ {\isacharparenleft}{\isasymAnd}x{\isachardot}\ P\ x\ {\isasymLongrightarrow}\ wf\ {\isacharparenleft}R\ x{\isacharparenright}{\isacharparenright}\ {\isasymLongrightarrow}\ wf\ {\isacharparenleft}same{\isacharunderscore}fst\ P\ R{\isacharparenright}%
114 is known to the well-foundedness prover of \isacommand{recdef}.
115 Thus well-foundedness of the given relation is immediate.
116 Furthermore, each recursive call descends along the given relation:
117 the first argument stays unchanged and the second one descends along
118 \isa{step{\isadigit{1}}\ f}. The proof merely requires unfolding of some definitions.
120 Normally you will then derive the following conditional variant of and from
121 the recursion equation%
123 \isacommand{lemma}\ {\isacharbrackleft}simp{\isacharbrackright}{\isacharcolon}\isanewline
124 \ \ {\isachardoublequote}wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}\ {\isasymLongrightarrow}\ find{\isacharparenleft}f{\isacharcomma}x{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ f\ x\ {\isacharequal}\ x\ then\ x\ else\ find{\isacharparenleft}f{\isacharcomma}\ f\ x{\isacharparenright}{\isacharparenright}{\isachardoublequote}\isanewline
125 \isacommand{by}\ simp%
126 \begin{isamarkuptext}%
127 \noindent and then disable the original recursion equation:%
129 \isacommand{declare}\ find{\isachardot}simps{\isacharbrackleft}simp\ del{\isacharbrackright}%
130 \begin{isamarkuptext}%
131 We can reason about such underdefined functions just like about any other
132 recursive function. Here is a simple example of recursion induction:%
134 \isacommand{lemma}\ {\isachardoublequote}wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}\ {\isasymlongrightarrow}\ f{\isacharparenleft}find{\isacharparenleft}f{\isacharcomma}x{\isacharparenright}{\isacharparenright}\ {\isacharequal}\ find{\isacharparenleft}f{\isacharcomma}x{\isacharparenright}{\isachardoublequote}\isanewline
135 \isacommand{apply}{\isacharparenleft}induct{\isacharunderscore}tac\ f\ x\ rule{\isacharcolon}find{\isachardot}induct{\isacharparenright}\isanewline
136 \isacommand{apply}\ simp\isanewline
138 \isamarkupsubsubsection{The {\tt\slshape while} combinator%
141 \begin{isamarkuptext}%
142 If the recursive function happens to be tail recursive, its
143 definition becomes a triviality if based on the predefined \isaindexbold{while}
144 combinator. The latter lives in the library theory
145 \isa{While_Combinator}, which is not part of \isa{Main} but needs to
146 be included explicitly among the ancestor theories.
148 Constant \isa{while} is of type \isa{{\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ bool{\isacharparenright}\ {\isasymRightarrow}\ {\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isacharparenright}\ {\isasymRightarrow}\ {\isacharprime}a}
149 and satisfies the recursion equation \begin{isabelle}%
150 \ \ \ \ \ while\ b\ c\ s\ {\isacharequal}\ {\isacharparenleft}if\ b\ s\ then\ while\ b\ c\ {\isacharparenleft}c\ s{\isacharparenright}\ else\ s{\isacharparenright}%
152 That is, \isa{while\ b\ c\ s} is equivalent to the imperative program
154 x := s; while b(x) do x := c(x); return x
156 In general, \isa{s} will be a tuple (better still: a record). As an example
157 consider the tail recursive variant of function \isa{find} above:%
159 \isacommand{constdefs}\ find{\isadigit{2}}\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}{\isacharparenleft}{\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isacharparenright}\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isachardoublequote}\isanewline
160 \ \ {\isachardoublequote}find{\isadigit{2}}\ f\ x\ {\isasymequiv}\isanewline
161 \ \ \ fst{\isacharparenleft}while\ {\isacharparenleft}{\isasymlambda}{\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}{\isachardot}\ x{\isacharprime}\ {\isasymnoteq}\ x{\isacharparenright}\ {\isacharparenleft}{\isasymlambda}{\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}{\isachardot}\ {\isacharparenleft}x{\isacharprime}{\isacharcomma}f\ x{\isacharprime}{\isacharparenright}{\isacharparenright}\ {\isacharparenleft}x{\isacharcomma}f\ x{\isacharparenright}{\isacharparenright}{\isachardoublequote}%
162 \begin{isamarkuptext}%
164 The loop operates on two ``local variables'' \isa{x} and \isa{x{\isacharprime}}
165 containing the ``current'' and the ``next'' value of function \isa{f}.
166 They are initalized with the global \isa{x} and \isa{f\ x}. At the
167 end \isa{fst} selects the local \isa{x}.
169 This looks like we can define at least tail recursive functions
170 without bothering about termination after all. But there is no free
171 lunch: when proving properties of functions defined by \isa{while},
172 termination rears its ugly head again. Here is
173 \isa{while{\isacharunderscore}rule}, the well known proof rule for total
174 correctness of loops expressed with \isa{while}:
176 \ \ \ \ \ {\isasymlbrakk}P\ s{\isacharsemicolon}\ {\isasymAnd}s{\isachardot}\ {\isasymlbrakk}P\ s{\isacharsemicolon}\ b\ s{\isasymrbrakk}\ {\isasymLongrightarrow}\ P\ {\isacharparenleft}c\ s{\isacharparenright}{\isacharsemicolon}\isanewline
177 \ \ \ \ \ \ \ \ {\isasymAnd}s{\isachardot}\ {\isasymlbrakk}P\ s{\isacharsemicolon}\ {\isasymnot}\ b\ s{\isasymrbrakk}\ {\isasymLongrightarrow}\ Q\ s{\isacharsemicolon}\ wf\ r{\isacharsemicolon}\isanewline
178 \ \ \ \ \ \ \ \ {\isasymAnd}s{\isachardot}\ {\isasymlbrakk}P\ s{\isacharsemicolon}\ b\ s{\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isacharparenleft}c\ s{\isacharcomma}\ s{\isacharparenright}\ {\isasymin}\ r{\isasymrbrakk}\isanewline
179 \ \ \ \ \ {\isasymLongrightarrow}\ Q\ {\isacharparenleft}while\ b\ c\ s{\isacharparenright}%
180 \end{isabelle} \isa{P} needs to be
181 true of the initial state \isa{s} and invariant under \isa{c}
182 (premises 1 and 2).The post-condition \isa{Q} must become true when
183 leaving the loop (premise 3). And each loop iteration must descend
184 along a well-founded relation \isa{r} (premises 4 and 5).
186 Let us now prove that \isa{find{\isadigit{2}}} does indeed find a fixed point. Instead
187 of induction we apply the above while rule, suitably instantiated.
188 Only the final premise of \isa{while{\isacharunderscore}rule} is left unproved
189 by \isa{auto} but falls to \isa{simp}:%
191 \isacommand{lemma}\ lem{\isacharcolon}\ {\isachardoublequote}{\isasymlbrakk}\ wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}{\isacharsemicolon}\ x{\isacharprime}\ {\isacharequal}\ f\ x\ {\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isasymexists}y\ y{\isacharprime}{\isachardot}\isanewline
192 \ \ \ while\ {\isacharparenleft}{\isasymlambda}{\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}{\isachardot}\ x{\isacharprime}\ {\isasymnoteq}\ x{\isacharparenright}\ {\isacharparenleft}{\isasymlambda}{\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}{\isachardot}\ {\isacharparenleft}x{\isacharprime}{\isacharcomma}f\ x{\isacharprime}{\isacharparenright}{\isacharparenright}\ {\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}y{\isacharcomma}y{\isacharprime}{\isacharparenright}\ {\isasymand}\isanewline
193 \ \ \ y{\isacharprime}\ {\isacharequal}\ y\ {\isasymand}\ f\ y\ {\isacharequal}\ y{\isachardoublequote}\isanewline
194 \isacommand{apply}{\isacharparenleft}rule{\isacharunderscore}tac\ P\ {\isacharequal}\ {\isachardoublequote}{\isasymlambda}{\isacharparenleft}x{\isacharcomma}x{\isacharprime}{\isacharparenright}{\isachardot}\ x{\isacharprime}\ {\isacharequal}\ f\ x{\isachardoublequote}\ \isakeyword{and}\isanewline
195 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ r\ {\isacharequal}\ {\isachardoublequote}inv{\isacharunderscore}image\ {\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}\ fst{\isachardoublequote}\ \isakeyword{in}\ while{\isacharunderscore}rule{\isacharparenright}\isanewline
196 \isacommand{apply}\ auto\isanewline
197 \isacommand{apply}{\isacharparenleft}simp\ add{\isacharcolon}inv{\isacharunderscore}image{\isacharunderscore}def\ step{\isadigit{1}}{\isacharunderscore}def{\isacharparenright}\isanewline
199 \begin{isamarkuptext}%
200 The theorem itself is a simple consequence of this lemma:%
202 \isacommand{theorem}\ {\isachardoublequote}wf{\isacharparenleft}step{\isadigit{1}}\ f{\isacharparenright}\ {\isasymLongrightarrow}\ f{\isacharparenleft}find{\isadigit{2}}\ f\ x{\isacharparenright}\ {\isacharequal}\ find{\isadigit{2}}\ f\ x{\isachardoublequote}\isanewline
203 \isacommand{apply}{\isacharparenleft}drule{\isacharunderscore}tac\ x\ {\isacharequal}\ x\ \isakeyword{in}\ lem{\isacharparenright}\isanewline
204 \isacommand{apply}{\isacharparenleft}auto\ simp\ add{\isacharcolon}find{\isadigit{2}}{\isacharunderscore}def{\isacharparenright}\isanewline
206 \begin{isamarkuptext}%
207 Let us conclude this section on partial functions by a
208 discussion of the merits of the \isa{while} combinator. We have
209 already seen that the advantage (if it is one) of not having to
210 provide a termintion argument when defining a function via \isa{while} merely puts off the evil hour. On top of that, tail recursive
211 functions tend to be more complicated to reason about. So why use
212 \isa{while} at all? The only reason is executability: the recursion
213 equation for \isa{while} is a directly executable functional
214 program. This is in stark contrast to guarded recursion as introduced
215 above which requires an explicit test \isa{x\ {\isasymin}\ dom\ f} in the
216 function body. Unless \isa{dom} is trivial, this leads to a
217 definition which is either not at all executable or prohibitively
218 expensive. Thus, if you are aiming for an efficiently executable definition
219 of a partial function, you are likely to need \isa{while}.%
224 %%% TeX-master: "root"