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  1. #include <cstdio>
  2.  
  3. template<class T>
  4. struct maybe
  5. {
  6.     maybe( const T& t ) : argument( t ), valid( true ) {}
  7.     maybe() : argument(), valid( false ) {}
  8.  
  9.     T argument;
  10.     bool valid;
  11. };
  12.  
  13. template<class T>
  14. maybe<T> just( const T& t ) { return maybe<T>(t); }
  15.  
  16. template<class T>
  17. maybe<T> nothing() { return maybe<T>(); }
  18.  
  19. auto terminal_maybe = [] ( auto term ) {
  20.     return [=] ( auto func ) {
  21.         return func( term );
  22.     };
  23. };
  24.  
  25. auto fmap_maybe = [] ( auto f ) {
  26.     return [=] ( auto t ) {
  27.         if( t.valid ) {
  28.             try {
  29.                 t.argument = f( t.argument );
  30.                 printf("argument = %d\n",t.argument);
  31.             }
  32.             catch(...) {
  33.                 t.valid = false;
  34.             }
  35.         }
  36.  
  37.         return (t.valid) ? terminal_maybe( just( t.argument ) ) : terminal_maybe( nothing<decltype(t.argument)>() );
  38.     };
  39. };
  40.  
  41. int main( int argc, char* argv[] )
  42. {
  43.     auto plus_2 = [] ( auto arg ) { return arg + 2; };
  44.     auto minus_2 = [] ( auto arg ) { return arg - 2; };
  45.  
  46.     maybe<int> forty = just(40);
  47.  
  48.     auto result = terminal_maybe(forty)
  49.         (fmap_maybe( plus_2 ))
  50.         (fmap_maybe( plus_2 ));
  51.  
  52.     result([] (maybe<int> m) { 
  53.     	if(m.valid) {
  54. 			printf("results = %d\n", m.argument);   		
  55.     	} else {
  56.     		printf("results = invalid\n");	
  57.     	}
  58.     } );
  59.  
  60.     return 0;
  61. }
Success #stdin #stdout 0s 3096KB
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Standard input is empty
stdout
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argument = 42
argument = 44
results = 44
https://ideone.com/DaCiA2
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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