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# class Method

`Method` objects are created by `Object#method`, and are associated with a particular object (not just with a class). They may be used to invoke the method within the object, and as a block associated with an iterator. They may also be unbound from one object (creating an `UnboundMethod`) and bound to another.

    class Thing
      def square(n)
        n*n
      end
    end
    thing = Thing.new
    meth  = thing.method(:square)

    meth.call(9)                 #=> 81
    [ 1, 2, 3 ].collect(&meth)   #=> [1, 4, 9]

    [ 1, 2, 3 ].each(&method(:puts)) #=> prints 1, 2, 3

    require 'date'
    %w[2017-03-01 2017-03-02].collect(&Date.method(:parse))
    #=> [#<Date: 2017-03-01 ((2457814j,0s,0n),+0s,2299161j)>, #<Date: 2017-03-02 ((2457815j,0s,0n),+0s,2299161j)>]

### Public Instance Methods

self << g → a_proc
Source

    static VALUE
    rb_method_compose_to_left(VALUE self, VALUE g)
    {
        g = to_callable(g);
        self = method_to_proc(self);
        return proc_compose_to_left(self, g);
    }

Returns a proc that is the composition of the given `g` and this method.

The returned proc takes a variable number of arguments. It first calls `g` with the arguments, then calls `self` with the return value of `g`.

    def f(ary) = ary << 'in f'

    f = self.method(:f)
    g = proc { |ary| ary << 'in proc' }
    (f << g).call([]) # => ["in proc", "in f"]

meth == other_meth → true or false
Source

    static VALUE
    method_eq(VALUE method, VALUE other)
    {
        struct METHOD *m1, *m2;
        VALUE klass1, klass2;

        if (!rb_obj_is_method(other))
            return Qfalse;
        if (CLASS_OF(method) != CLASS_OF(other))
            return Qfalse;

        Check_TypedStruct(method, &method_data_type);
        m1 = (struct METHOD *)RTYPEDDATA_GET_DATA(method);
        m2 = (struct METHOD *)RTYPEDDATA_GET_DATA(other);

        klass1 = method_entry_defined_class(m1->me);
        klass2 = method_entry_defined_class(m2->me);
        if (RB_TYPE_P(klass1, T_ICLASS)) klass1 = RBASIC_CLASS(klass1);
        if (RB_TYPE_P(klass2, T_ICLASS)) klass2 = RBASIC_CLASS(klass2);

        if (!rb_method_entry_eq(m1->me, m2->me) ||
            klass1 != klass2 ||
            m1->klass != m2->klass ||
            m1->recv != m2->recv) {
            return Qfalse;
        }

        return Qtrue;
    }

Two method objects are equal if they are bound to the same object and refer to the same method definition and the classes defining the methods are the same class or module.

Also aliased as: eql?
method === obj → result_of_method

Invokes the meth with the specified arguments, returning the method’s return value.

    m = 12.method("+")
    m.call(3)    #=> 15
    m.call(20)   #=> 32

Using `Method#===` allows a method object to be the target of a `when` clause in a case statement.

    require 'prime'

    case 1373
    when Prime.method(:prime?)
      # ...
    end

Alias for: call
self >> g → a_proc
Source

    static VALUE
    rb_method_compose_to_right(VALUE self, VALUE g)
    {
        g = to_callable(g);
        self = method_to_proc(self);
        return proc_compose_to_right(self, g);
    }

Returns a proc that is the composition of this method and the given `g`.

The returned proc takes a variable number of arguments. It first calls `self` with the arguments, then calls `g` with the return value of `self`.

    def f(ary) = ary << 'in f'

    f = self.method(:f)
    g = proc { |ary| ary << 'in proc' }
    (f >> g).call([]) # => ["in f", "in proc"]

meth[args, ...] → obj

Invokes the meth with the specified arguments, returning the method’s return value.

    m = 12.method("+")
    m.call(3)    #=> 15
    m.call(20)   #=> 32

Using `Method#===` allows a method object to be the target of a `when` clause in a case statement.

    require 'prime'

    case 1373
    when Prime.method(:prime?)
      # ...
    end

Alias for: call
arity → integer
Source

    static VALUE
    method_arity_m(VALUE method)
    {
        int n = method_arity(method);
        return INT2FIX(n);
    }

Returns an indication of the number of arguments accepted by a method. Returns a nonnegative integer for methods that take a fixed number of arguments. For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required arguments. Keyword arguments will be considered as a single additional argument, that argument being mandatory if any keyword argument is mandatory. For methods written in C, returns -1 if the call takes a variable number of arguments.

    class C
      def one;    end
      def two(a); end
      def three(*a);  end
      def four(a, b); end
      def five(a, b, *c);    end
      def six(a, b, *c, &d); end
      def seven(a, b, x:0); end
      def eight(x:, y:); end
      def nine(x:, y:, **z); end
      def ten(*a, x:, y:); end
    end
    c = C.new
    c.method(:one).arity     #=> 0
    c.method(:two).arity     #=> 1
    c.method(:three).arity   #=> -1
    c.method(:four).arity    #=> 2
    c.method(:five).arity    #=> -3
    c.method(:six).arity     #=> -3
    c.method(:seven).arity   #=> -3
    c.method(:eight).arity   #=> 1
    c.method(:nine).arity    #=> 1
    c.method(:ten).arity     #=> -2

    "cat".method(:size).arity      #=> 0
    "cat".method(:replace).arity   #=> 1
    "cat".method(:squeeze).arity   #=> -1
    "cat".method(:count).arity     #=> -1

box → box or nil
Source

    static VALUE
    method_box(VALUE obj)
    {
        struct METHOD *data;
        const rb_box_t *box;

        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
        box = data->me->def->box;
        if (!box) return Qnil;
        if (box->box_object) return box->box_object;
        rb_bug("Unexpected box on the method definition: %p", (void*) box);
        UNREACHABLE_RETURN(Qnil);
    }

Returns the `Ruby::Box` where `meth` is defined in.

call(args, ...) → obj
Source

    static VALUE
    rb_method_call_pass_called_kw(int argc, const VALUE *argv, VALUE method)
    {
        return rb_method_call_kw(argc, argv, method, RB_PASS_CALLED_KEYWORDS);
    }

Invokes the meth with the specified arguments, returning the method’s return value.

    m = 12.method("+")
    m.call(3)    #=> 15
    m.call(20)   #=> 32

Using `Method#===` allows a method object to be the target of a `when` clause in a case statement.

    require 'prime'

    case 1373
    when Prime.method(:prime?)
      # ...
    end

Also aliased as: ===, []
clone → new_method
Source

    static VALUE
    method_clone(VALUE self)
    {
        VALUE clone;
        struct METHOD *orig, *data;

        TypedData_Get_Struct(self, struct METHOD, &method_data_type, orig);
        clone = TypedData_Make_Struct(CLASS_OF(self), struct METHOD, &method_data_type, data);
        rb_obj_clone_setup(self, clone, Qnil);
        RB_OBJ_WRITE(clone, &data->recv, orig->recv);
        RB_OBJ_WRITE(clone, &data->klass, orig->klass);
        RB_OBJ_WRITE(clone, &data->iclass, orig->iclass);
        RB_OBJ_WRITE(clone, &data->owner, orig->owner);
        RB_OBJ_WRITE(clone, &data->me, rb_method_entry_clone(orig->me));
        return clone;
    }

Returns a clone of this method.

    class A
      def foo
        return "bar"
      end
    end

    m = A.new.method(:foo)
    m.call # => "bar"
    n = m.clone.call # => "bar"

curry → proc
curry(arity) → proc
Source

    static VALUE
    rb_method_curry(int argc, const VALUE *argv, VALUE self)
    {
        VALUE proc = method_to_proc(self);
        return proc_curry(argc, argv, proc);
    }

Returns a curried proc based on the method. When the proc is called with a number of arguments that is lower than the method’s arity, then another curried proc is returned. Only when enough arguments have been supplied to satisfy the method signature, will the method actually be called.

The optional arity argument should be supplied when currying methods with variable arguments to determine how many arguments are needed before the method is called.

    def foo(a,b,c)
      [a, b, c]
    end

    proc  = self.method(:foo).curry
    proc2 = proc.call(1, 2)          #=> #<Proc>
    proc2.call(3)                    #=> [1,2,3]

    def vararg(*args)
      args
    end

    proc = self.method(:vararg).curry(4)
    proc2 = proc.call(:x)      #=> #<Proc>
    proc3 = proc2.call(:y, :z) #=> #<Proc>
    proc3.call(:a)             #=> [:x, :y, :z, :a]

eql?(other_meth) → true or false

Two method objects are equal if they are bound to the same object and refer to the same method definition and the classes defining the methods are the same class or module.

Alias for: ==
hash → integer
Source

    static VALUE
    method_hash(VALUE method)
    {
        struct METHOD *m;
        st_index_t hash;

        TypedData_Get_Struct(method, struct METHOD, &method_data_type, m);
        hash = rb_hash_start((st_index_t)m->recv);
        hash = rb_hash_method_entry(hash, m->me);
        hash = rb_hash_end(hash);

        return ST2FIX(hash);
    }

Returns a hash value corresponding to the method object.

See also `Object#hash`.

inspect → string
Source

    static VALUE
    method_inspect(VALUE method)
    {
        struct METHOD *data;
        VALUE str;
        const char *sharp = "#";
        VALUE mklass;
        VALUE defined_class;

        TypedData_Get_Struct(method, struct METHOD, &method_data_type, data);
        str = rb_sprintf("#<% "PRIsVALUE": ", rb_obj_class(method));

        mklass = data->iclass;
        if (!mklass) mklass = data->klass;

        if (RB_TYPE_P(mklass, T_ICLASS)) {
            /* TODO: I'm not sure why mklass is T_ICLASS.
             * UnboundMethod#bind() can set it as T_ICLASS at convert_umethod_to_method_components()
             * but not sure it is needed.
             */
            mklass = RBASIC_CLASS(mklass);
        }

        if (data->me->def->type == VM_METHOD_TYPE_ALIAS) {
            defined_class = data->me->def->body.alias.original_me->owner;
        }
        else {
            defined_class = method_entry_defined_class(data->me);
        }

        if (RB_TYPE_P(defined_class, T_ICLASS)) {
            defined_class = RBASIC_CLASS(defined_class);
        }

        if (UNDEF_P(data->recv)) {
            // UnboundMethod
            rb_str_buf_append(str, rb_inspect(defined_class));
        }
        else if (RCLASS_SINGLETON_P(mklass)) {
            VALUE v = RCLASS_ATTACHED_OBJECT(mklass);

            if (UNDEF_P(data->recv)) {
                rb_str_buf_append(str, rb_inspect(mklass));
            }
            else if (data->recv == v) {
                rb_str_buf_append(str, rb_inspect(v));
                sharp = ".";
            }
            else {
                rb_str_buf_append(str, rb_inspect(data->recv));
                rb_str_buf_cat2(str, "(");
                rb_str_buf_append(str, rb_inspect(v));
                rb_str_buf_cat2(str, ")");
                sharp = ".";
            }
        }
        else {
            mklass = data->klass;
            if (RCLASS_SINGLETON_P(mklass)) {
                VALUE v = RCLASS_ATTACHED_OBJECT(mklass);
                if (!(RB_TYPE_P(v, T_CLASS) || RB_TYPE_P(v, T_MODULE))) {
                    do {
                       mklass = RCLASS_SUPER(mklass);
                    } while (RB_TYPE_P(mklass, T_ICLASS));
                }
            }
            rb_str_buf_append(str, rb_inspect(mklass));
            if (defined_class != mklass) {
                rb_str_catf(str, "(% "PRIsVALUE")", defined_class);
            }
        }
        rb_str_buf_cat2(str, sharp);
        rb_str_append(str, rb_id2str(data->me->called_id));
        if (data->me->called_id != data->me->def->original_id) {
            rb_str_catf(str, "(%"PRIsVALUE")",
                        rb_id2str(data->me->def->original_id));
        }
        if (data->me->def->type == VM_METHOD_TYPE_NOTIMPLEMENTED) {
            rb_str_buf_cat2(str, " (not-implemented)");
        }

        // parameter information
        {
            VALUE params = rb_method_parameters(method);
            VALUE pair, name, kind;
            const VALUE req = ID2SYM(rb_intern("req"));
            const VALUE opt = ID2SYM(rb_intern("opt"));
            const VALUE keyreq = ID2SYM(rb_intern("keyreq"));
            const VALUE key = ID2SYM(rb_intern("key"));
            const VALUE rest = ID2SYM(rb_intern("rest"));
            const VALUE keyrest = ID2SYM(rb_intern("keyrest"));
            const VALUE block = ID2SYM(rb_intern("block"));
            const VALUE nokey = ID2SYM(rb_intern("nokey"));
            int forwarding = 0;

            rb_str_buf_cat2(str, "(");

            if (RARRAY_LEN(params) == 3 &&
                RARRAY_AREF(RARRAY_AREF(params, 0), 0) == rest &&
                RARRAY_AREF(RARRAY_AREF(params, 0), 1) == ID2SYM('*') &&
                RARRAY_AREF(RARRAY_AREF(params, 1), 0) == keyrest &&
                RARRAY_AREF(RARRAY_AREF(params, 1), 1) == ID2SYM(idPow) &&
                RARRAY_AREF(RARRAY_AREF(params, 2), 0) == block &&
                RARRAY_AREF(RARRAY_AREF(params, 2), 1) == ID2SYM('&')) {
                forwarding = 1;
            }

            for (int i = 0; i < RARRAY_LEN(params); i++) {
                pair = RARRAY_AREF(params, i);
                kind = RARRAY_AREF(pair, 0);
                if (RARRAY_LEN(pair) > 1) {
                    name = RARRAY_AREF(pair, 1);
                }
                else {
                    // FIXME: can it be reduced to switch/case?
                    if (kind == req || kind == opt) {
                        name = rb_str_new2("_");
                    }
                    else if (kind == rest || kind == keyrest) {
                        name = rb_str_new2("");
                    }
                    else if (kind == block) {
                        name = rb_str_new2("block");
                    }
                    else if (kind == nokey) {
                        name = rb_str_new2("nil");
                    }
                    else {
                        name = Qnil;
                    }
                }

                if (kind == req) {
                    rb_str_catf(str, "%"PRIsVALUE, name);
                }
                else if (kind == opt) {
                    rb_str_catf(str, "%"PRIsVALUE"=...", name);
                }
                else if (kind == keyreq) {
                    rb_str_catf(str, "%"PRIsVALUE":", name);
                }
                else if (kind == key) {
                    rb_str_catf(str, "%"PRIsVALUE": ...", name);
                }
                else if (kind == rest) {
                    if (name == ID2SYM('*')) {
                        rb_str_cat_cstr(str, forwarding ? "..." : "*");
                    }
                    else {
                        rb_str_catf(str, "*%"PRIsVALUE, name);
                    }
                }
                else if (kind == keyrest) {
                    if (name != ID2SYM(idPow)) {
                        rb_str_catf(str, "**%"PRIsVALUE, name);
                    }
                    else if (i > 0) {
                        rb_str_set_len(str, RSTRING_LEN(str) - 2);
                    }
                    else {
                        rb_str_cat_cstr(str, "**");
                    }
                }
                else if (kind == block) {
                    if (name == ID2SYM('&')) {
                        if (forwarding) {
                            rb_str_set_len(str, RSTRING_LEN(str) - 2);
                        }
                        else {
                            rb_str_cat_cstr(str, "...");
                        }
                    }
                    else {
                        rb_str_catf(str, "&%"PRIsVALUE, name);
                    }
                }
                else if (kind == nokey) {
                    rb_str_buf_cat2(str, "**nil");
                }

                if (i < RARRAY_LEN(params) - 1) {
                    rb_str_buf_cat2(str, ", ");
                }
            }
            rb_str_buf_cat2(str, ")");
        }

        { // source location
            VALUE loc = rb_method_location(method);
            if (!NIL_P(loc)) {
                rb_str_catf(str, " %"PRIsVALUE":%"PRIsVALUE,
                            RARRAY_AREF(loc, 0), RARRAY_AREF(loc, 1));
            }
        }

        rb_str_buf_cat2(str, ">");

        return str;
    }

Returns a human-readable description of the underlying method.

    "cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
    (1..3).method(:map).inspect    #=> "#<Method: Range(Enumerable)#map()>"

In the latter case, the method description includes the “owner” of the original method (`Enumerable` module, which is included into `Range`).

`inspect` also provides, when possible, method argument names (call sequence) and source location.

    require 'net/http'
    Net::HTTP.method(:get).inspect
    #=> "#<Method: Net::HTTP.get(uri_or_host, path=..., port=...) <skip>/lib/ruby/2.7.0/net/http.rb:457>"

`...` in argument definition means argument is optional (has some default value).

For methods defined in C (language core and extensions), location and argument names can’t be extracted, and only generic information is provided in form of `*` (any number of arguments) or `_` (some positional argument).

    "cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
    "cat".method(:+).inspect       #=> "#<Method: String#+(_)>""

Also aliased as: to_s
name → symbol
Source

    static VALUE
    method_name(VALUE obj)
    {
        struct METHOD *data;

        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
        return ID2SYM(data->me->called_id);
    }

Returns the name of the method.

original_name → symbol
Source

    static VALUE
    method_original_name(VALUE obj)
    {
        struct METHOD *data;

        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
        return ID2SYM(data->me->def->original_id);
    }

Returns the original name of the method.

    class C
      def foo; end
      alias bar foo
    end
    C.instance_method(:bar).original_name # => :foo

owner → class_or_module
Source

    static VALUE
    method_owner(VALUE obj)
    {
        struct METHOD *data;
        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
        return data->owner;
    }

Returns the class or module on which this method is defined. In other words,

    meth.owner.instance_methods(false).include?(meth.name) # => true

holds as long as the method is not removed/undefined/replaced, (with private_instance_methods instead of instance_methods if the method is private).

See also `Method#receiver`.

    (1..3).method(:map).owner #=> Enumerable

parameters → array
Source

    static VALUE
    rb_method_parameters(VALUE method)
    {
        return method_def_parameters(rb_method_def(method));
    }

Returns the parameter information of this method.

    def foo(bar); end
    method(:foo).parameters #=> [[:req, :bar]]

    def foo(bar, baz, bat, &blk); end
    method(:foo).parameters #=> [[:req, :bar], [:req, :baz], [:req, :bat], [:block, :blk]]

    def foo(bar, *args); end
    method(:foo).parameters #=> [[:req, :bar], [:rest, :args]]

    def foo(bar, baz, *args, &blk); end
    method(:foo).parameters #=> [[:req, :bar], [:req, :baz], [:rest, :args], [:block, :blk]]

receiver → object
Source

    static VALUE
    method_receiver(VALUE obj)
    {
        struct METHOD *data;

        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
        return data->recv;
    }

Returns the bound receiver of the method object.

    (1..3).method(:map).receiver # => 1..3

source_location → [String, Integer, Integer, Integer, Integer]
Source

    VALUE
    rb_method_location(VALUE method)
    {
        return method_def_location(rb_method_def(method));
    }

Returns the location where the method was defined. The returned `Array` contains:

    (1) the Ruby source filename
    (2) the line number where the definition starts
    (3) the column number where the definition starts
    (4) the line number where the definition ends
    (5) the column number where the definitions ends

This method will return `nil` if the method was not defined in Ruby (i.e. native).

super_method → method
Source

    static VALUE
    method_super_method(VALUE method)
    {
        const struct METHOD *data;
        VALUE super_class, iclass;
        ID mid;
        const rb_method_entry_t *me;

        TypedData_Get_Struct(method, struct METHOD, &method_data_type, data);
        iclass = data->iclass;
        if (!iclass) return Qnil;
        if (data->me->def->type == VM_METHOD_TYPE_ALIAS && data->me->defined_class) {
            super_class = RCLASS_SUPER(rb_find_defined_class_by_owner(data->me->defined_class,
                data->me->def->body.alias.original_me->owner));
            mid = data->me->def->body.alias.original_me->def->original_id;
        }
        else {
            super_class = RCLASS_SUPER(RCLASS_ORIGIN(iclass));
            mid = data->me->def->original_id;
        }
        if (!super_class) return Qnil;
        me = (rb_method_entry_t *)rb_callable_method_entry_with_refinements(super_class, mid, &iclass);
        if (!me) return Qnil;
        return mnew_internal(me, me->owner, iclass, data->recv, mid, rb_obj_class(method), FALSE, FALSE);
    }

Returns a `Method` of superclass which would be called when super is used or nil if there is no method on superclass.

to_proc → proc
Source

    static VALUE
    method_to_proc(VALUE method)
    {
        VALUE procval;
        rb_proc_t *proc;

        /*
         * class Method
         *   def to_proc
         *     lambda{|*args|
         *       self.call(*args)
         *     }
         *   end
         * end
         */
        procval = rb_block_call(rb_mRubyVMFrozenCore, idLambda, 0, 0, bmcall, method);
        GetProcPtr(procval, proc);
        proc->is_from_method = 1;
        return procval;
    }

Returns a `Proc` object corresponding to this method.

to_s → string

Returns a human-readable description of the underlying method.

    "cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
    (1..3).method(:map).inspect    #=> "#<Method: Range(Enumerable)#map()>"

In the latter case, the method description includes the “owner” of the original method (`Enumerable` module, which is included into `Range`).

`inspect` also provides, when possible, method argument names (call sequence) and source location.

    require 'net/http'
    Net::HTTP.method(:get).inspect
    #=> "#<Method: Net::HTTP.get(uri_or_host, path=..., port=...) <skip>/lib/ruby/2.7.0/net/http.rb:457>"

`...` in argument definition means argument is optional (has some default value).

For methods defined in C (language core and extensions), location and argument names can’t be extracted, and only generic information is provided in form of `*` (any number of arguments) or `_` (some positional argument).

    "cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
    "cat".method(:+).inspect       #=> "#<Method: String#+(_)>""

Alias for: inspect
unbind → unbound_method
Source

    static VALUE
    method_unbind(VALUE obj)
    {
        VALUE method;
        struct METHOD *orig, *data;

        TypedData_Get_Struct(obj, struct METHOD, &method_data_type, orig);
        method = TypedData_Make_Struct(rb_cUnboundMethod, struct METHOD,
                                       &method_data_type, data);
        RB_OBJ_WRITE(method, &data->recv, Qundef);
        RB_OBJ_WRITE(method, &data->klass, Qundef);
        RB_OBJ_WRITE(method, &data->iclass, orig->iclass);
        RB_OBJ_WRITE(method, &data->owner, orig->me->owner);
        RB_OBJ_WRITE(method, &data->me, rb_method_entry_clone(orig->me));

        return method;
    }

Dissociates meth from its current receiver. The resulting `UnboundMethod` can subsequently be bound to a new object of the same class (see `UnboundMethod`).