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The try-catch-finally statement provides means for dealing with exceptions that may occur in a given block of code.

<vipbnf><TryCatchTerm>:
    try <Term> <CatchFinally>-list end try

<CatchFinally>: one of
    catch <Variable> do <Trap>-<Handler>
    finally <Finally>-<Handler>

<Handler>:
    <Term></vipbnf>

A try-construction thus have a <vpbnf>Term</vpbnf> and a list of catch and finally handlers.

The <vpbnf><Term></vpbnf> and the handlers are not allowed to leave backtrack points (i.e. they cannot have mode <vp>multi</vp> or <vp>nondeterm</vp>).

A try-construction with more than one handler is equivalent to nesting several try-constructions with one handler each. I.e. the following term:

<vipbnf>try
   <Body>
catch <Var1> do
   <Handler1>
finally
   <Handler2>
catch <Var2> do
    <Handler3>
end try</vipbnf>

Is equivalent to these nested terms:

<vipbnf>try
    try
        try
            <Body>
        catch <Var1> do
            <Handler1>
        end try
    finally
        <Handler2>
    end try
catch <Var2> do
    <Handler3>
end try</vipbnf>

==== try-catch ====

Consider the try-catch construction

<vipbnf>try
    <Body>
catch <Var> do
    <Handler>
end try</vipbnf>

First <vpbnf><Body></vpbnf> is evaluated.

If <vpbnf><Body></vpbnf> fails or succeeds the whole try construction fails or succeeds, respectively.  I.e. if <vpbnf><Body></vpbnf> does not terminate with an exception the try construction corresponds to evaluating <vpbnf><Body></vpbnf>.

If <vpbnf><Body></vpbnf> terminates with an exception, then the exception is caught by the catch handler.  Meaning that first <vpbnf><Var></vpbnf> is bound to the exception (in PFC context the exception will be a <vp>traceId</vp>) and then <vpbnf><Handler></vpbnf> is evaluated.  In this case the construction behaves as if <vpbnf><Handler></vpbnf> is evaluated with <vpbnf><Var></vpbnf> bound to the caught exception.

Notice, that no bindings are made by <vpbnf><Body></vpbnf> if it terminates with an exception.

{{Example| Handle the situation where a file cannot be read:

<vip>clauses
    read(Filename) = Contents :-
        try
            Contents = file::readFile(Filename)
        catch TraceId do
            stdio::writef("Could not read the file: %\n", Filename),
            exceptionDump::dumpToStdio(TraceId),
            Contents = ""
        end try.</vip>

First <vp>Contents = file::readFile(Filename)</vp> is evaluated, if that does not terminate with an exception <vp>read</vp> returns the contents of the file.

If it raises an exception the handler is evaluated with <vp>TraceId</vp> bound to the exception.  So in this case a message is written to <vp>stdio</vp> and then the exception is dumped to <vp>stdio</vp>, and finally <vp>Contents</vp> is set to the empty string which is returned as result.
}}

==== try-finally ====

Consider the try-finally construction:

<vipbnf>try
    <Body>
finally
    <Handler>
end try</vipbnf>

The purpose of the construction is to evaluate the <vpbnf><Handler></vpbnf> after the <vpbnf><Body></vpbnf> no matter how the <vpbnf><Body></vpbnf> terminates, i.e. whether it succeeds, fails or terminates with an exception (it cannot leave backtrack points).

The evaluation is like this

First <vpbnf><Body></vpbnf> is evaluated.

* If <vpbnf><Body></vpbnf> succeeds: <vpbnf><Handler></vpbnf> is executed and the try-finally construction succeeds.
* If <vpbnf><Body></vpbnf> fails: <vpbnf><Handler></vpbnf> is executed and the try-finally construction fails.
* If <vpbnf><Body></vpbnf> terminates with an exception <vpbnf><Exn></vpbnf>: <vpbnf><Handler></vpbnf> is executed and the try-finally construction terminates with the exception <vpbnf><Exn></vpbnf>.

{{Example| Ensure that an <vp>odbcStatement</vp> is <vp>free</vp>'d after use:

<vip>clauses
    tryGetName(Connection, PersonId) = Name :-
        Stmt = odbcStatement::new(Connection),
        try
            Stmt:setParameterValue(1, core::integer(PersonId)),
            Stmt:execDirect("select name from person where id=?"),
            Stmt:fetch(),
            Name = Stmt:getParameterValue_string(1)
        finally
            Stmt:free()
        end try.</vip>

The <vp>Stmt</vp> is <vp>free</vp>'d also if <vp>fetch</vp> fails, or if something terminates with an exception.
}}
<noinclude>[[Category:Language Reference Subarticle]]</noinclude>

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