Java模式匹配库 JMatch

     The JMatch language extends Java with pattern matching that supports both data abstraction and iteration abstraction. Patterns are not tied to algebraic data constructors as in ML; instead, a single JMatch method may be used in several modes, some of which can serve as patterns. JMatch provides modal abstraction that simplifies the specification and implementation of abstract data types. These modes that may share a common implementation as a boolean formula. Thus, the specification, implementation, and use of iteration abstractions are made convenient, by automatically finding multiple solutions to a formula or pattern.    <br />    <img title="jmatch-logo-small.gif" border="0" alt="jmatch-logo-small.gif" src="https://simg.open-open.com/show/80dc18bad737d5a422f22faafc8465a6.gif" width="125" height="130" />    <br />    <br />    <span style="font-weight:bold;">项目地址</span>：    <a href="/misc/goto?guid=4958197869188397636" target="_blank">http://www.cs.cornell.edu/Projects/jmatch/</a>    <br />    <br /> 示例代码：    <pre class="brush:java; toolbar: true; auto-links: false;">Balancing a red-black tree with pattern matching    static RBNode balance(int color, int value, RBTree left, RBTree right)  {    if (color == BLACK) {      switch (value,left,right) {        case int z,             RBNode(RED,int y,RBNode(RED,int x,RBTree a,RBTree b),RBTree c),             RBTree d:        case z, RBNode(RED,x,a,RBNode(RED,y,b,c)),d:        case x, a, RBNode(RED,z,RBNode(RED,y,b,c),d):        case x, a, RBNode(RED,y,b,RBNode(RED,z,c,d)):          return RBNode(RED,y,RBNode(BLACK,x,a,b),RBNode(BLACK,z,c,d));      }    }    return RBNode(color, value, left, right);  }    A modal abstraction  Containment predicate is also a tree iterator    boolean contains(Object elem) {    if (left != null && value < elem) return left.contains(elem);    if (value.equals(elem)) return true;    if (right != null && value > elem) return right.contains(elem);  } iterates(elem) {    foreach (left != null && left.contains(Object e)) yield e;    yield value;    foreach (right != null && right.contains(Object e)) yield e;  }    One formula implements both modes: concise, ensures consistency    boolean contains(Object elem) iterates(elem) (    (left != null && elem < value && left.contains(elem)) ||    elem = value ||    (right != null && elem > value && right.contains(elem))  )    A client loop selecting elements matching a pattern    Set t = new RedBlackTree(); ...  foreach (t.contains(Point(int x, x*2))) {       // get all objects in set that are points (x,y) where y is twice x; bind x      print(x);  }    Invertible CPS conversion using pattern matching    // Forward: convert(e) is the CPS conversion of e.  // Backward: e is the expression for which result is the CPS conversion.  static public Expr convert(Expr e) returns(e) (    Var k = fresh("k", e) && (      e = Var(String v1) &&      result = Lambda(k, Apply(k, e))    else      e = Lambda(Var v2, Expr body) &&      result = Lambda(k, Apply(k,                 Lambda(v2,                   Lambda(k, Apply(convert(body), k)))))    else      e = Apply(Expr fn, Expr arg) &&      Var f = fresh("f", arg) &&      result = Lambda(k, Apply(convert(fn),                 Lambda(f, Apply(convert(arg),                   Lambda(Var("v"), Apply(Apply(f, Var("v")), k))))))    )  )    // Forward: whether v is free in e.  // Backward: all v's that are free in e.  static boolean FV(Expr e, String v) iterates(v) (    e = Var(v) ||    e = Lambda(String v1, Expr body) && FV(body, v) && !(v = v1) ||    e = Apply(Expr fn, Expr arg) && (FV(fn, v) || FV(arg, v))  )    // Forward: a variable name that is fresh in e and starts with x.  // Backward: whether result is not free in e.  static Var fresh(String base, Expr e) {    boolean dummy = true;    for (int i = 0; dummy; i++) {        String var = base + i;        if (!FV(e, var)) return Var(var);    }  }    returns() ( !FV(e, result.name) )  ...  Expr church_one = lambda("f", lambda("x", apply("f", "x")));  Expr cps_one = CPS.convert(church_one);  let CPS.convert(Expr e) = cps_one // e equals church_one!</pre>