type holder

Dao (programming language)

Dao is an object-oriented scripting language with dynamically typed variables supporting complex data structures. It has powerful text processing abilities, such as regular expression matching. It provides a wealth of built-in numerical data types such as complex numbers and multi-dimensional numeric arrays, as well as their corresponding arithmetic operations. Support for multi-threaded programming is also an integrated part of Dao. The Dao interpreter is implemented as a lightweight and efficient virtual register machine (Dao VM) in standard C. The Dao VM can be easily extended with C or C++, through a simple and transparent interface.

Advanced features of the language include:

  • A mixed static and dynamic type system with automatic type deduction capabilities
  • A flexible macro system that allows definition of new syntax
  • Concurrent and distributed programming with message passing interfaces
  • Direct embedding of C code into Dao programs


Hello World

The classic hello world program can be written as follows:

stdio.print ("Hello world!" )

Here stdio is the standard library to handle input and output. In Dao, there are no built-in functions; all functions are part of a certain library, such as stdio, stdlib, math, coroutine, reflect, mpi (message passing interface), network, and thread.

The stdlib.listmeth routine can be used to display the methods in a library. The methods in math, for example, can be displayed using:

 stdlib.listmeth (math )

Data Types

A Dao variable can be implicitly declared by assigning the result of an expression to a variable name.

 a_number = 123
 a_string = "abc"
a_list = { 1, 2, 3 } a_map = { "CD"=>123, "AB"=>456 }
 a_tuple1 = (123, "ABC" )
 a_tuple2 = (index => 123, name => "ABC" ) # tuple with named items
 a_vector = [1, 2, 3 ]
 a_matrix = [1, 2; 3, 4 ]

By default, a variable will have a fixed type that is inferred from the expression that is used to declare the variable; this is called implicit typing. The type of a variable can also be specified explicitly, using the following Pascal-style declaration syntax:

 var_typed : type
 var_typed : type = value

type can be one of the following built-in types: int, float, double, string, complex, list, map, tuple, array, buffer, routine, or some composition of these types.

For example:

 a_list2 : list<list<float> >
a_list3 : list<list<string> > = {} a_map2 : map<string,int> = { "ABC"=>123 }
 a_tuple1 : tuple<int,string> = (123, "ABC" )
 a_tuple2 : tupleint,name:string> = (index => 123, name => "ABC" )

type can also be a Dao class name or the name of a user-defined C type. Special keywords for typing include: any for any type, ? for undefined types and @X for a type holder that can be initialized to a certain type in a parameter list.

All typed variables undergo static checking.

Control Flow


 a = math.rand();
 if(a > 0.75 ){
    a -= 0.75
}elif(a > 0.5 ){
    a -= 0.5
    a -= 0.25


 i = 0;
 while(i < 10 ) i++


 for(i=0; i<10; i++ ) stdio.println(i )
 for(i=0 : 9 ) stdio.println(i )
 for(i=0 : 2 : 9 ) stdio.println(i )


a_list = { 1, 2, 3 }

 for(it in a_list ) stdio.println(it )
a_list2 = { "a", "b", "c" }
 for(it in a_list; it2 in a_list2 ) stdio.println(it, it2 )

For-in also works for hash.


 a = "abc"
 switch(a ){
   case 1 : a += 123;
   case "abc" : a += "456"
   default : stdio.println("default case" )


The declaration of a basic function (known in Dao as routines) looks like this:

 routine foo(a, b : string, c = 123 ) => int
    return 456;

Dao supports first-order functions. The definition of such functions is identical to that of normal function definition, with the following differences:

  • there is no need for a function name, but the created function must be assigned to a variable;
  • the default value expressions do not need to be constant expressions, as they are evaluated at run time when the function is created;
  • the function body may contain variables defined in the "upper" function that creates it.

 a = "ABC";
 rout = routine(x, y : string, z = a+a ){
    a += "_abc";
    stdio.println("lambda ", a )
    stdio.println("lambda ", y )
    stdio.println("lambda ", z )
 rout(1, "XXX" );


 class MyNumber(value = 0 )
   my Value = 0; # default value is zero
   Value = value;
routine setValue(value : int ){ Value = value } routine getValue(){ return Value } }


Dao macros are defined in the following way:


This example enables "while-do-end" without brackets:

    while $EXP do [$BL ] end
}as{ while($EXP ){ [$BL ] } }

Concurrent and Distributed Programming

Asynchronous Function Call

Probably the simplest way to create multi-threaded programs in Dao is to use asynchronous function calls (AFC). The way of using AFC is almost identical to that of normal function calls, with the exception that the keyword async must follow the call.

 myfunc(myparams ) async;
 myobj.mymeth(myparams ) async;

Any functions or methods can be invoked in such an asynchronous way.

Normally AFC is executed in a separated native thread, which can be either an idle thread available from the thread pool, or a new thread created on the fly.

Message Passing Interface

With Message Passing Interface APIs provided in library mpi, one can easily do concurrent and distributed programming in Dao. In the MPI library, there are three principal functions: spawn(), send() and receive(). With spawn(), one can create lightweight virtual machine processes or real operation system processes, in the local or remote computers; and with send() and receive(), a process can send messages to or receive messages from other processes.

The process names are of the following form:


"@real_process@@hostname" identifies an operating system process on host "@@hostname", and "virtual_process@real_process@@hostname" identifies a virtual process within "@real_process@@hostname". Message passing can only happen among virtual processes. Each real process has a main virtual process named "self". Any part can be omitted. For example, when "virtual_process" is omitted, it means the "self" virtual process, and "@@hostname" alone identifies the "self" within the operating system process that binds to port 4115 (D : 4, A : 1, O : 15 ).

 # spawn a real process to run script "mpi_script.dao"
 mpi.spawn("@pid", "mpi_script.dao" );
 # spawn a virtual process to run function "test"
 mpi.spawn("vmp@pid", "test" );

 # send message to a real process
 mpi.send("@pid", "TO MAIN" );

 # print a received message
 stdio.println(mpi.receive() );

 # spawn a real process on "@@localhost" to run script "mpi_script.dao"
 mpi.spawn("@pid2@@localhost", "mpi_script.dao" );
 # spawn a virtual process on "@pid2@@localhost" to run function "test"
 mpi.spawn("vmp@pid2@@localhost", "test" );
 # send message to the virtual process "vmp@pid2@@localhost"
 mpi.send("vmp@pid2@@localhost", "ANOTHER", 123.456 );b

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