case_

Synopsis

template <class Exp, class C1, ..., class Cn>
struct case_
{
  // unspecified
};

Description

It implements the case construct of functional programming languages. The classes C1 ... Cn have to be instances one of the following templates:

template <class Pattern, class Body>
struct matches;

template <class Pattern, class Body>
struct matches_c;

The arguments of both matches have to be syntaxes. When case_ is evaluated, the angyl-bracket expression Exp is evaluated and matched against the Pattern patterns in order. When it matches one of them, the Body belonging to it is returned. When the expression doesn't match any of the patterns, an exception is returned with a no_case_matched value. When the result of Exp matches multiple patterns, the first one is used. The open variables of a Pattern can be used in the Body belonging to it.

The maximum number of matches elements is limited by the value of MPLLIBS_LIMIT_CASE_SIZE macro.

Header

#include <mpllibs/metamonad/case_.hpp>

Expression semantics

For any m, n > 0, e, p1, ..., pn, b1, ..., bn, q1, ..., qm, c1, ..., cm angly-bracket expressions, assuming that e matches all of q1, ..., qm and does not match any of p1, ..., pn, the following are equivalent:

matches_c<p1, b1>
matches<syntax<p1>, syntax<b1>>

case_<
  e,
  matches<p1, b1>,
  // ...
  matches<pn, bn>
>::type
exception<...>

case_<
  e,
  matches<q1, c1>,
  // ...
  matches<qm, cm>
>::type
match_let<q1, e, c1>::type

case_<
  e,
  matches<p1, b1>,
  // ...
  matches<pn, bn>,
  matches<q1, c1>,
  // ...
  matches<qm, cm>
>::type
match_let<q1, e, c1>::type

Example

using namespace mpllibs::metamonad::name;

template <class A>
struct maybe_something :
  boost::mpl::if_<
    typename boost::is_same<A, int>::type,
    just<double>,
    nothing
  >
{};

typedef
  eval_syntax<
    case_< maybe_something<int>::type,
      matches_c<just<var<x>>, returns<x>>,
      matches_c<nothing,      nothing>,
      matches_c<_,            nothing>
    >
  >::type
  this_is_double;

Copyright Abel Sinkovics (abel at elte dot hu) 2011. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt

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