fantasy-states/state.js

State control structure.

var Tuple2 = require('fantasy-tuples').Tuple2,
    daggy = require('daggy'),
    State = daggy.tagged('run');

// Methods
State.of = function(a) {
    return State(function(b) {
        return Tuple2(a, b);
    });
};
State.prototype.chain = function(f) {
    var state = this;
    return State(function(s) {
        var result = state.run(s);
        return f(result._1).run(result._2);
    });
};
State.get = State(function(s) {
    return Tuple2(s, s);
});
State.modify = function(f) {
    return State(function(s) {
        return Tuple2(null, f(s));
    });
};
State.put = function(s) {
    return State.modify(function(a) {
        return s;
    });
};
State.prototype.evalState = function(s) {
    return this.run(s)._1;
};
State.prototype.exec = function(s) {
    return this.run(s)._2;
};

// Derived
State.prototype.map = function(f) {
    return this.chain(function(a) {
        return State.of(f(a));
    });
};
State.prototype.ap = function(a) {
    return this.chain(function(f) {
        return a.map(f);
    });
};

// Transformer
State.StateT = function(M) {
    var StateT = daggy.tagged('run');
    StateT.lift = function(m) {
        return StateT(function(b) {
            return m.map(function(c) {
                return Tuple2(c, b);
            });
        });
    };
    StateT.of = function(a) {
        return StateT(function(b) {
            return M.of(Tuple2(a, b));
        });
    };
    StateT.prototype.chain = function(f) {
        var state = this;
        return StateT(function(s) {
            var result = state.run(s);
            return result.chain(function(t) {
                return f(t._1).run(t._2);
            });
        });
    };
    StateT.get = StateT(function(s) {
        return M.of(Tuple2(s, s));
    });
    StateT.modify = function(f) {
        return StateT(function(s) {
            return M.of(Tuple2(null, f(s)));
        });
    };
    StateT.put = function(s) {
        return StateT.modify(function(a) {
            return s;
        });
    };
    StateT.prototype.evalState = function(s) {
        return this.run(s).map(function(t) {
            return t._1;
        });
    };
    StateT.prototype.exec = function(s) {
        return this.run(s).map(function(t) {
            return t._2;
        });
    };
    StateT.prototype.map = function(f) {
        return this.chain(function(a) {
            return StateT.of(f(a));
        });
    };
    StateT.prototype.ap = function(a) {
        return this.chain(function(f) {
            return a.map(f);
        });
    };
    return StateT;
};

// Export
if(typeof module != 'undefined')
    module.exports = State;