GScript

GScript is my attempt at yet another scripting language. It started with obviously too ambitous goal of supporting static typing, which proved to be too much. This is still a goal, but not one that I hope to achieve in the nearest future.

GScript is implemented in Java because of the following reasons:

• it was meant for use on Android
• Java has reflection
• Java has garbage collector

Building

Requirements

• Java
• maven

Script in bin/ directory are shell scripts, so at least the following tools are required:

• functioning bash
• functioning gawk

Script gsdot (and gssvg and gsdot, by transition) requires:

• functioning dot tool (so GraphViz)

Compilation

GScript can be built using maven utility. To build everything, run:

mvn package

Usage

Few example scripts are in directory:

progs/basic/

Not all of them work, as these scripts are used for (regression) testing.

Running scripts

bin/gsr -f progs/basic/test0094.gs

Dumping "assembly"

bin/gsda progs/basic/test0094.gs out.asm

NOTE: Adding .asm at the end of output file is not necessary, but VIM tries to highlight syntax and does pretty decend job.

Examplary program:

def main() {
        def x = List[ 1, 2, 3, 4, 5 ];
        return x[3];
}

after decompilation yields:

.object.constant
        0 java.lang.Integer 1
.object.constant
        1 java.lang.Integer 2
.object.constant
        2 java.lang.Integer 3
.object.constant
        3 java.lang.Integer 4
.object.constant
        4 java.lang.Integer 5

.object.function main
// DBG: .file test
       0 PUSHCONST 0                         // DBG: 2,16-2,16
       2 PUSHCONST 1                         // DBG: 2,19-2,19
       4 PUSHCONST 2                         // DBG: 2,22-2,22
       6 PUSHCONST 3                         // DBG: 2,25-2,25
       8 PUSHCONST 4                         // DBG: 2,28-2,28
      10 MKLIST 5                            // DBG: 2,10-2,30
      12 GETVAR 0                            // DBG: 2,2-2,6
      14 SET                                 // DBG: 2,2-2,30
      15 POP                                 // DBG: 2,2-2,31
      16 GETVAR 0                            // DBG: 3,9-3,9
      18 PUSHCONST 2                         // DBG: 3,11-3,11
      20 SUBSCRIPT                           // DBG: 3,9-3,12
      21 END                                 // DBG: 3,2-3,13
      22 END                                 // DBG: 1,1-4,1

This is, unfortunately, unidirectional operation - there is no way to generate executable code from this output.

Generating PNG or SVG files

There are three scripts that generate files with graphical AST representation:

bin/gspng FILE, which redirects to bin/gsdot --type png --file FILE

bin/gssvg FILE, which redirects to bin/gsdot --type svg --file FILE

Exemplary code used above yields the following graph in SVG format:

gspng

bin/gspng progs/basic/test0094.gs

gssvg

bin/gssvg progs/basic/test0094.gs

gsdot

bin/gsdot --type png --file progs/basic/test0094.gs

Features

Neater features of GScript include:

Lambda with variables capturing

Capturing is done the way Java does it, that is captured variable keeps its value from the moment it was captured. This was initially done to perform "proper closures" (I think so), that is if value in external scope is changed then lambda also observes this change.

Awkward example of lambda with capture is in test0027.gs. Another contrived example is in test0032.gs. Passing lambdas around is in test0054.gs.

"Functions as first class citizens"

Functions can be assigned to variables and passed around. Example in test0041.gs.

Constant folding

This is sometimes successful, for example in test0024.gs, and sometimes less so, for example in test0025.gs.

First example successfuly reduces whole expression to 0 constant, but second one fails to notice that x and y are, in fact, constants. I suppose that noticing it would be trivial if only SSA was used, but it isn't. For now.

Range based for loops

For loops can operate on ranges, with syntax:

for (x | iterable)
    ....

Example in test0083.gs.