examples: show how to create and pass a V string to a V function, and…

… then get back a V string in Python

examples/call_v_from_python/test.py

@@ -1,12 +1,26 @@
 from ctypes import *
 import math, os
 
+## Load the V shared library:
 so_file="./test.so"
 if os.name=="nt":
   so_file="./test.dll"
 lib = CDLL(so_file)
+
+## Pass an integer to a V function, and receiving back an integer:
 print("lib.square(10) result is", lib.square(10))
 assert lib.square(10) == 100, "Cannot validate V square()."
 
+## Pass a floating point number to a V function:
 lib.sqrt_of_sum_of_squares.restype = c_double
 assert lib.sqrt_of_sum_of_squares(c_double(1.1), c_double(2.2)) == math.sqrt(1.1*1.1 + 2.2*2.2), "Cannot validate V sqrt_of_sum_of_squares()."
+
+## Passing a V string to a V function, and receiving back a V string:
+class VString(Structure):
+  _fields_ = [("str", c_char_p), ("len", c_int)]
+
+lib.process_v_string.argtypes = [VString]
+lib.process_v_string.restype = VString
+
+assert lib.process_v_string(VString(b'World', 5)).str == b'v World v'
+print('Hello', str(lib.process_v_string(VString(b'World', 5)).str, 'utf-8'))

examples/call_v_from_python/test.v

@@ -1,5 +1,6 @@
 module test
 
+// Note: compile this with `v -d no_backtrace -shared test.v`
 import math
 
 @[export: 'square']
@@ -11,3 +12,9 @@ fn square(i int) int {
 fn sqrt_of_sum_of_squares(x f64, y f64) f64 {
 	return math.sqrt(x * x + y * y)
 }
+
+// you do not have to use the same name in the export attribute
+@[export: 'process_v_string']
+fn work(s string) string {
+	return 'v ${s} v'
+}