How to for-loop in MOE SVL function? - moe-svl

Question

In MOE SVL, I keep trying to create a basic for-loop by following the Control Flow documentation:
#svl
global function main []
for a = 10, 6, -1 loop
pr a;
endloop
endfunction
When I run the script:
moebatch -load for_loop.svl -exec "main[]"
I get the error:
ERROR: failed to load '/home/sean/svl_test/for_loop.svl'
'/home/sean/svl_test/for_loop.svl' line 5: 'a' is undefined
Error evaluating expression 'main[]'
'main' is undefined
How am I supposed to define a in the loop?

Answer 1

When executing commands from the SVL REPL you don't need to declare variables. Therefore, when writing a loop in the prompt like this:
for a = 10, 6, -1 loop pr a; endloop
The interpreter does not complain. If you write the same loop in a function within a script however, each variable needs to be declare first. The declaration can be inside the loop but must be before its first reference:
#svl
local function main []
local a;
for a = 10, 6, -1 loop
local b = 2;
pr [a, a-b];
endloop
endfunction
When you write a script in SVL there are two types of scripts:
Load scripts
These are scripts you load to extent the available functions within MOE. All functions declared as global can be called either from the REPL or within other functions in other scripts. For example if you write a script called add3script.svl like this:
#svl
global function add3 [a,b,c]
return a + b + c;
endfunction
You can load this functions into MOE (GUI or moebatch) and call it. To use it in moebatch directly you can do:
moebatch -load add3script.svl -exec "pr add3[1,2,3]"
This command will lunch the terminal version of MOE, load the SVL script add3script.svl which makes the add3 function available because it is set to global. Then we execute the command pr add3[1,2,3] which prints 6 and then moebatch quits. You can also launch moebatch to bring up the REPL and load the script:
moebatch -load add3script.svl
On the REPL the add3 function is now available. To load a script in the GUI version of MOE you can click MOE | File | Open then navigate to the add3script.svl and choose from the 'Action' dropdown on the File/Open panel 'Load SVL File'. Then click OK. Now add3 is available in the SVL command prompt in the GUI version of MOE.
An SVL script can contain multiple global and local functions but when loaded only the global functions can be used from outside the script. For example if we change add3script.svl to this:
#svl
local function printResult value
pr value;
endfunction
global function add3 [a,b,c]
printResult [a + b + c];
endfunction
global function mul3 [a,b,c]
printResult [a * b * c];
endfunction
You can use add3 and mul3 when loaded like mentioned above but printResult can only be called from within add3script.svl.
Run scripts
Instead of loading a script to make its functions available to use outside the script a script can be also be executed (or run). This allows to execute for example a workflow which is self contained within the script. This could be useful to write a GUI application. By default when an SVL script is executed as a run file the interpreter looks for a function called main and executes its instructions (helloworld.svl):
#svl
local function main []
pr 'Hello World!';
endfunction
This script could be executed from the terminal like this:
moebatch -run helloworld.svl
This prints 'Hello World!' and terminates. If another function then main should be executed a special instruction can be added:
#svl
#set main 'altmain'
local function gettext []
return 'Alt main text';
endfunction
local function altmain []
pr gettext[];
endfunction
local function main []
pr 'Hello World!';
endfunction
If you run this script it will print 'Alt main text' since the main function is set to altmain which calls the local function gettext.
To run such functions within GUI MOE you can simply double click on the SVL script.

Answer 2

It is not possible to define a in the loop declaration, as one might expect from C++ or Python. Instead, a local variable must be defined before the loop can be executed:
#svl
global function main []
local a;
for a = 10, 6, -1 loop
pr a;
endloop
endfunction
This is counter-intuitive, because loop code in the documentation does not do this, because it is only inside of functions that variables must be declared before usage. In the global scope, for example when executing code in the MOE SVL REPL, the looping variable does not need to be declared beforehand.