🔒 Translating some java into python

[torpid haven]

I know pretty much nothing about java and im struggling to translate it into python, some help would be appreciated so i can get going with this interpreter.

[grim forge]

@torpid haven

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[torpid haven]

my code so far:

class Lox:
    @staticmethod
    def main()

https://craftinginterpreters.com/scanning.html#the-interpreter-framework

thats the book i am following along with

public class Lox {
  public static void main(String[] args) throws IOException {
    if (args.length > 1) {
      System.out.println("Usage: jlox [script]");
      System.exit(64); 
    } else if (args.length == 1) {
      runFile(args[0]);
    } else {
      runPrompt();
    }
  }
}

i need some help with the parameters of the main function

and the throw which i assume is error handling

[gleaming kiln]

why do you even have a class for that

that's not how you write Python code

just write that code directly in the script

[torpid haven]

the tutorial is in java, i want to write it in python instead?

[gleaming kiln]

if you want to write in Python, why are you following a Java tutorial?

[torpid haven]

because it was suggested by nedbat and its a good tutorial

[gleaming kiln]

are you a Python beginner?

[torpid haven]

not really but sort of

im trying to learn more about how programming languages work by building an interpreter

[gleaming kiln]

that's a pretty high level task for someone not yet comfortable with the language

and I believe there are tutorials for building interpreters in Python

[torpid haven]

are there any extremely good and informational ones? since the crafting interpreters one is pulling the topic apart very well

[gleaming kiln]

I'm afraid I haven't really looked

[torpid haven]

ah okay ill try asking in py discussion

[left patio]

Python does not oblige u to use class every where and u can start with a simple main function. Translating from one language to the other requires base knowledge for both languages and I think is not suitable for beginner

[shrewd haven]

that's a book for creating interpeters/compilers in general, they're not for teaching a specific language, it just so happen they use c and java. i've used that book and followed it along in both rust and swift before.

[shrewd haven]

you need to be aware that you shouldn't translate the code 1:1 because different languages work differently, it's not enough with just changing the syntax. python has no main entry point besides the file that's being run, that means whatever is in the main function of the java code can be in the top level of your main python file (main here indicating the python file you're invoking with the python interpreter)

i wouldn't really recommend this as a project for you to learn python, but rather for learning how to create an interpreter/compiler where you happen to already be familiar with python instead of c/java.

[torpid haven]

i am confident with python just not interpreters or java

[shrewd haven]

you can't write python like java, you have to write python like python

class Lox:
    @staticmethod
    def main():
        ...
```what will call this main function?

in java, it's called automatically

in python it's not, so you'd have to call it yourself```py
class Lox:
@staticmethod
def main():
...

Lox.main()

at which point, the class is entirely redundant

you may be confident, and i don't intend to tear that confidence, but you do clearly lack some foundational experience that tutorial will require you to know given you're not actually following their implementation but rather trying to interpret and write your own.

[torpid haven]

well i’ll be honest idk how else to learn

[shrewd haven]

if your goal is to learn how to create interpreters and/or compilers, why couldn't you just follow the tutorial in c or java?

if your goal is to learn python, then find a tutorial that teaches you python instead

if you intend to do both, start with the latter and work your way up to the former

[torpid haven]

i don’t want to learn java or c i want an interpreter built in python as that’s the language i wanna work in

[shrewd haven]

#1399708210285379616 message

[torpid haven]

yeah but what should i do to learn python more

[shrewd haven]

#1399708210285379616 message

[torpid haven]

i’m not new to python, i don’t know what to learn

[shrewd haven]

how and where the main execution point of your program is, is a basic starting block you should know, in python that'd be any script you directly invoke or a __main__ file, as opposed to most AoT or JIT compiled languages that have a special main function that may or may not have to be a method on a class. if you truly do think you have the required experience in python, you should be able to write your implementations without having to translate the java/c code given the actual details are explained in text (the book is language agnostic, they only use java/c as examples). even so, if you did have the required python experience, that should be enough to cross over to java/c where you can at least be able to filter out the bits you know you'd need in python (which is the logic code) and the bits you'd know wont translate to python (java/c specific features).

[torpid haven]

i dont know what else to do then

this is the problem i have

what do i do if i dont have the experience to learn it

i cant program anything because i dont know it all

and to learn it i need to do it, but i cant

[shrewd haven]

start with something smaller?

[torpid haven]

like what?

[shrewd haven]

anything you find interesting

[torpid haven]

this is what i find interesting

[shrewd haven]

write a simple parser then?

you're going from 0 to 100 real quick by attempting to write your own interpreter

[torpid haven]

well thats what this tutorial was supposed to be but its just the whole interpreter instead

i asked for help on where to find out how i can write a parser and this is what i got sent

[shrewd haven]

this seems like a nice little getting started resource https://tomassetti.me/parsing-in-python/ then pick something simpler to parse, like math expressions, an ini file parser, csv parser, etc...

[torpid haven]

thank you

[torpid haven]

is there anyway to do it without a library

[shrewd haven]

yes, that resource goes through how

with a bunch of tools/libraries you could benefit from if you want

[torpid haven]

where is the actual tutorial? this just seems like its listing libraries that you can use

[shrewd haven]

idk how else to help you man it literally goes through the fundamental things you need to understand about parsers

scroll down to

Useful Things To Know About Parsers

there are plenty of other resources, you could try googling till you find something better

[tender tree]

Crafting Interpreters is the best resource I'm aware of for learning how to make your own programming language. I'd suggest investing the effort to get good enough at Java to at least follow the code examples in the book

[shrewd haven]

that's what i suggested in an essence but fallbacked to recommending something smaller like a csv parser - they don't seem to be interested in investing some time in java or c

[crude dock]

you don't need to translate the code line by line, but it's important that you know how to write code equivalent to what they're showing.

there's a list of lox interpreters written in Python listed here: https://github.com/munificent/craftinginterpreters/wiki/Lox-implementations#python, if you're really stuck on something maybe referencing what one of these have done could help

[torpid haven]

might just make a calculator at this point 😭

[tender tree]

The Python equivalent of this is: ```py
import sys
if len(sys.argv) > 2:
print("Usage: jlox [script]")
sys.exit(64)
elif len(sys.argv) == 2:
run_file(sys.argv[1])
else:
run_prompt()

[shrewd haven]

actually that's a good start, you'll have to parse math expressions

[torpid haven]

ive made like 3 calculators before

1 terminal and 2 gui

[crude dock]

!projects has a lot of things you can check out as well if you need some ideas

[grim forge]

Kindling Projects

The Kindling projects page contains a list of projects and ideas programmers can tackle to build their skills and knowledge.

[grizzled stratus]

with actual parsing logic? i.e. PEMDAS rules, etc?

[crude dock]

do they handle precedence? (e.g will (5+2*3) give you the right answer)

[torpid haven]

no, im trying to make a maths expression parser that makes an AST now

but lowkey might just give up on that too

[grizzled stratus]

okay, so your thread is about actually doing a proper calculator?

[tender tree]

Genuinely, you'll notice that the Python version and the Java version are extremely similar. Python and Java are very similar languages

Don't be afraid to learn something new

[torpid haven]

import sys

equation = sys.argv
if len(equation) > 1:
    print("Usage: parser [equation]")
elif len(equation) == 1:
    pass
else:
    pass

tokens = equation.split(sep=None, maxsplit=-1)

thats what i did

i just want it to do like 8+2

but first i gotta make it an AST

[grizzled stratus]

lemme see if I can find something

I remember Guido himself wrote something about parsing with the new PEG parser and a special form of packratt parsing (memoization) that allows left-recursion ... but I think that one is too advanced.

[torpid haven]

just wanna learn parsing

cant find anything helpful

[grizzled stratus]

I'll see what I can find.

[shrewd haven]

i found this video if it'd be in interest https://www.youtube.com/watch?v=88lmIMHhYNs

seems to be a series of 4 videos in total though

[torpid haven]

thanks ill have a look

[grizzled stratus]

it looks decent, though it's very dense.

[torpid haven]

what do you mean by dense?

i think this is a great start though

[grizzled stratus]

very densely packed.

[torpid haven]

definitely can use this and then take it and modify it

[grizzled stratus]

the information doesn't go into much details or reasonings for specific things, as far as I can tell

but I suppose that can be fine.

[torpid haven]

i mean i've been doing some research myself

i had a look at iter()

since i wasnt familiar with it

[slender ibex]

For extended reading, search shunting yard algorithm

[steel hazel]

Don't look down on calculators. Take it far enough, that's a tiny language you have right there.

I recommend a recursive descent parser. It's efficient (O(n) on input size) and very flexible to work with. You can add operators, modify precedences and change associativity just by tweaking a few numbers.

Check my project (link in bio) if you want to see how it works.

[torpid haven]

your calculator is a billion times harder than mine

i just wanna add single digit numbers

[steel hazel]

Then you don't need a parser!

I thought you want to learn to write a parser.

[torpid haven]

i dont even know what i want to do anymore

im just bored

i've had 5 weeks off school and start a new one in 5 weeks

im so bored

[torpid haven]

that is what i wanted

ive literally just sat around everyday looking for something to do

[steel hazel]

Maybe instead of wanting something to do, you should be wanting to do something.

[torpid haven]

thats what im trying thats why im here

but everythings just too difficult

and the basic stuff isnt gonna help me

[steel hazel]

Ramp up slowly.

First, write something that does multiple digit integers, + and *

31 + 42 * 5 + 6

OK you have something to do, go do it.

[torpid haven]

how

this is my problem

[steel hazel]

What is the difficulty for you in this task?

[torpid haven]

well i dont know how to get my computer to understand what each number is and what to do with symbols

since it doesnt know ive got to tell it

[steel hazel]

How much Python do you know? Are you able to do string manipulation? Do you know how to get a number from a string?

[torpid haven]

could you iter() through it and then use the types of each character

[gleaming bolt]

@torpid haven there are a lot of implementations of lox in python you can take a look at them

[torpid haven]

im not fussed about lox right now really

i just wanna know the basic stuff

[gleaming bolt]

https://github.com/munificent/craftinginterpreters/wiki/Lox-implementations#python

[lavish valley]

im not sure why would you explicitly use iter, just loop over the characters in the string, keep a variable to add related characters into, "dump" it each time you meet something that logically "splits" tokens (an operator, or a parentheses), and skip whitespace

[torpid haven]

does iter() not loop over the characters?

[lavish valley]

for parsing expressions with operator precedence, you can use a recursive approach with the precedence of the previous operator as an argument, so in cases like 2+3*5+6, when you meet the *, the next parse would know that * (an operator with higher precedence) was before it, therefore it should return early instead of binding

[torpid haven]

with next()

[lavish valley]

why would you explicitly use the iterator interface instead of just a for loop?
for character in string:

[torpid haven]

idk

[lavish valley]

this problem is over-blown-up by academic people trying to write "the most efficient" parsers, and it doesnt need any fancy terminology or years of experience
parsing is the smallest problem of language development, yet resources spend a lot of time on it

literally a "loop and go-left sometimes parser" is enough

[torpid haven]

sounds like im gonna have great fun with language development 🥹

tokens = []
DIGITS = "0123456789"

equation = input("Equation > ")
for character in equation:
    current_char = equation[character]
    if current_char in DIGITS:

thats what ive got

am i on the right track or no

[lavish valley]

character is already the character, not an index, so you dont need the [character] part
but yes, you should check what kind of character you have, and either add it to some buffer or split based on that

[torpid haven]

oh is it

for character in equation:
    if character in DIGITS:

[steel hazel]

Yes to this. With slight modifications to account for associativity.

[torpid haven]

tokens = []
DIGITS = "0123456789"

equation = input("Equation > ")
for character in equation:
    store = None
    decimal_count = 0
    if character in DIGITS:
        store += character
    elif character == ".":
        store += character
        if decimal_count > 1:
            raise TypeError(f"Illegal characters entered, a value contains 2 or more decimal points")

gonna go eat dinner

[lavish valley]

the slight modification is literally > vs >=, in this case
and a table for operator associativity when they have operators where that would matter (e.g. exponentiation)

[torpid haven]

but thats what i have right now

[lavish valley]

you're resetting store each character, so its never going to actually store anything

[torpid haven]

is it not adding it?

with +=

[lavish valley]

you have store = None at the start of each iteration

[torpid haven]

oh so i’ll define it outside

i need to get better at scopes i think

[steel hazel]

I have a different precedence level for entering a next call, vs while being in the call itself. Not quite as simple as < vs <=

But let's focus on helping OP

[steel hazel]

If I may suggest, you may want to ramp down a bit and work on basic Python exercises instead.

[torpid haven]

they’re boring and just me repeating myself

[torpid haven]

@lavish valley i managed to get this!

Equation > 2 + 3 - 6 * 12 / 9
['2', '+', '3', '-', '6', '*', '12', '/', '9']

thats my tokens list

[grim forge]

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