Cygnus Interpreter

A simple interpreter for a script language Cygnus, written in C# ##What is Cygnus? Cygnus is a procedure-oriented, light weight, and interactive programming language, written in C#. Users can easily extend its functions by importing C# functions, and maybe embed it into C# project.

Basic Type

Constant

Cygnus constant can represent integer, double, Boolean,and string. 
It doesn't support char type currently. 
To declare a string, you may use "" or '' to enclose its content.
Example 1:
	x = 12
	y = 23.1
	z = true
	greeting = 'Hello Cygnus!'

Array

Array in Cygnus could contain elements from different data types. 
To declare an array, you may use function 'array' or use '{', '}' to initialize your array.
The index of an array starts from zero, the same as in C#.
Example 2:
	arr = array(10)
	arr = {1,2,3,'nice','day'}
	print(a[3]+a[4])

List

List could also contain elements from various types. 
You need to apply function 'list' to have a new list. 
If you input an array into it, you can initialize your list with the elements in your array.
Example 3:
	mylist = list()
	mylist = list(1,2,3,'nice','day')
	print(mylist[0])

Dictionary

Dictionary is another important data type. 
The key of dictionary must be an constant, and no restriction for the data type of the values. 
To initialize a dictionary, you need use function 'dict'. 
If you input an array with key-value pairs, the dictionary will be initialized with these elements.
Example 4:
	mydict = dict()
	mydict = dict({'John',21},{'Marry',27})
	mydict['Martin'] = 35
	print(mydict['John'])

Control Statement

The grammar of the control statement is inspired by Lua, Python and Pascal. Every structure here needs an 'end' for closure.

If

In Cygnus, there are two types of decision structure. 
One is 'If-Then', and another is 'If-Then-Else'. 
There is no 'elseif' or 'elif' in Cygnus. 
Example 5:
	if a > b then 
		print(a)
	end
	
	if a > b then
		print(a)
	else
		print(b)
	end

While

While statement is really useful when you need a loop which will continuously execute under the condition given.
You can easily learn how to use while loop in Cygnus by the following example:
Example 6:
	i = 0
	while i < 10 do
		print(i)
		i = i + 1
	end

For

The 'for' loop in Cygnus is like 'foreach' in C#. 
The grammar is inspired by 'for' in Python. 
Let's  take a look at this example:
Example 7:
	for i in range(10) do
		print(i)
	end
The 'range' function here is a lazy function, which is the same as 'xrange' in Python. 
So don't worry about generating a large set for the loop, it won't :)
Also,  you may use the for loop to traverse every element in a collection.
Example 8:
	sum = 0
	for i in {1,2,3} do
		sum = sum + i
	end

Break

You may need a break if you want to jump out of a loop
Example 9:
	for i in range(10) do
		if i > 5 then
			break
		else
			print(i)
		end
	end

Return

Keyword 'return' can let you go out of the function. 
Refer to example 10.

Function

Define a function

Use keyword 'def' to define a function. 
Notice that the function has its own scope. 
The variables in the functions could be out of the functions' scope,
which means the function can contain global variables.
Example 10:
	def max(x, y) begin
		if x > y then
			return x
		else
			return y
		end
	end

Higher-order function

Function in Cygnus can be used as a parameter, 
namely you can write higher-order functions. 
See the following example:
Example 11:
	def mul(x,y) begin
		return x * y
	end
	def mul2(f,x) begin
		return f(x,2)
	end
	print(mul2(mul,10))
Nevertheless, the lambda expression is not supported in Cygnus currently.

Advanced Features

Table

The table in Cygnus are designed for being used as a mini class :), which enables users to define a new data type.
It is capable of inheriting, but it can only have no more than one parent table.
Example 12:
	p = table(age,job)
	p.age = 23
	p.job = 'doctor'
	setparent(p,table(city))
I recommend writing a constructor for the table, which makes the code more readable.
Example 13:
	def person(age,job) begin
		p = table(age,job)
		p.age = age
		p.job = job
		return p
	end
	p1 = person(23,'engineer')

Cygnus Code Examples

Find the oldest person

def person(name,age) begin 
	p = table(name,age)
	p.name = name
	p.age = age
	return p
end

people = {
	person('shao',23),
    person('wang',20),
    person('zhang',21),
    person('he',22)
}
max_age = -1
max_name = ''
for p in people do
	if max_age < p.age then 
    	max_age = p.age
    	max_name = p.name
	end
end
result = strformat('name: {0}  age: {1}',max_name,max_age)
print(result)

Create a linkedlist

def linkednode(value,previous,next) begin
	node = table(value,previous,next)
	node.value = value
	node.previous = previous
	node.next = next
	return node
end

def linkedlist() begin
	mylist = table(first,last,count)
	mylist.first = null
	mylist.last = null
	return mylist
end

def link_add(mylist,value) begin
	if mylist.first == null then
		node = linkednode(value,null,null)
		mylist.first = node
		mylist.last = node
	else
		node = linkednode(value,mylist.last,null)
		mylist.last.next = node
		mylist.last = node
	end
	return void
end

def link_disp(mylist) begin
	current = mylist.first
	while current != null do
		print(current.value)
		current = current.next
	end
	return void
end

mylist = linkedlist()
link_add(mylist,12)
link_add(mylist,24)
link_add(mylist,36)
link_add(mylist,48)
link_disp(mylist)

Built-in functions

Basic Functions

• print(object): print the object

• array(n): initialize an array

• list(args): initialize a list

• dict(args): initialize a dictionary

• table(args): initialize a table

• setparent(table,parent_table): set a table's parent table

• length(set): return the length of a set,e.g. array,list...

• import(dll_path,namespace_and_class): import C# functions from a dll file

• execfile(file_path): execute code from a file

• throw(error): throw an error

• delete(args): delete variables

• scan(str): print a sentence in the console, wait for the input, and return the input as string

• range(start, stop, step): generate a sequence

• exit(): exit from the programm