result.ps

%%% Result: [v T]
%%% Error: [e F]

/ok {                               % v
    [ exch true ]                   % [v T]
} def

/error {                            % e
    [ exch false ]                  % [e F]
} def

/map {                              % A {op} %%% {op} is operation v -> v' to be called for OK results.
    [ 3 1 roll                      % [ A {op}
    exch                            % [ {op} A
    aload pop                       % [ {op} x ? 
    {                               % [ {op} v % map v to op(v)
        exch                        % [ v {op}
        exec                        % [ op(v)
        true                        % [ op(v) T
    }                  
    {                               % [ {op} e % leave e as is
        exch pop false              % [ e F
    }
    ifelse
    ]                               % R
} def

/map-error {                        % A {op} %%% {op} is operation e -> e' to be called for Error results.
    [ 3 1 roll                      % [ A {op}
    exch                            % [ {op} A
    aload pop                       % [ {op} x ? 
    {                               % [ {op} v % leave v as is
        exch pop true               % [ v T 
    }                  
    {                               % [ {op} e % map e to op(e)
        exch                        % [ e {op}
        exec                        % [ op(e)
        false                       % [ op(e) F
    }
    ifelse
    ]                               % R
} def

/flatmap {                          % A {op} %%% {op} is operation v -> R to be called for OK results.
    exch                            % {op} A
    aload pop                       % {op} x ? 
    {                               % {op} v % map v to op(v)
        exch                        % v {op}
        exec                        % R=op(v)
    }                  
    {                               % {op} e % leave e as is
        exch pop                    % e
    }
    ifelse
} def