theory TimeLimit+
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imports "../Appendix"+
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begin+
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+
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section {* Restricting the Runtime of a Function\label{rec:timeout} *} +
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+
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text {*+
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  {\bf Problem:}+
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  Your tool should run only a specified amount of time.\smallskip+
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+
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  {\bf Solution:} In PolyML 5.2.1 and later, this can be achieved +
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  using the function @{ML timeLimit in TimeLimit}.\smallskip+
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+
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  Assume you defined the Ackermann function on the ML-level.+
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*}+
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+
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ML{*fun ackermann (0, n) = n + 1+
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  | ackermann (m, 0) = ackermann (m - 1, 1)+
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  | ackermann (m, n) = ackermann (m - 1, ackermann (m, n - 1)) *}+
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+
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text {*+
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+
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  Now the call +
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+
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  @{ML_response_fake [display,gray] "ackermann (4, 12)" "\<dots>"}+
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+
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  takes a bit of time before it finishes. To avoid this, the call can be encapsulated +
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  in a time limit of five seconds. For this you have to write+
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+
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@{ML_response_fake_both [display,gray]+
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"TimeLimit.timeLimit (Time.fromSeconds 5) ackermann (4, 12) +
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  handle TimeLimit.TimeOut => ~1"+
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"~1"}+
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+
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  where @{text TimeOut} is the exception raised when the time limit+
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  is reached.+
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+
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  Note that @{ML  "timeLimit" in TimeLimit} is only meaningful when you use PolyML 5.2.1+
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  or later, because this version of PolyML has the infrastructure for multithreaded +
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  programming on which @{ML "timeLimit" in TimeLimit} relies.+
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+
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\begin{readmore}+
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   The function @{ML "timeLimit" in TimeLimit} is defined in the structure+
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  @{ML_struct TimeLimit} which can be found in the file +
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  @{ML_file "Pure/ML-Systems/multithreading_polyml.ML"}.+
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\end{readmore}+
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+
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 +
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*}+
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end+
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