14a Reading from source

[ngjunsiang]

Before we embark on File IO, it feels kind of strange to do so when our developers have to edit the source code of our main program to insert their code ... frankly it would be embarrassing to see this in real-world code!

So let's at least put something in place for us to read our source code from an external source file. For now, we'll just use main.pseudo as the filename of this source file (we don't support importing code from multiple source files anyway).

And let's read in the source code from that file at the start:
https://github.com/nyjc-computing/pseudo/blob/8d87a0983ec887b1e16d5fb4bac5fd6668bfd206/main.py#L12-L14

[ngjunsiang]

What if the user has a different file they want to use? It's common to pass that as the first argument to our interpreter as follows:

main.py myotherfile.pseudo

(Right now our file is called main.py. We'll give it a cooler name when we're packaging it.)

In python, we can extract these arguments passed to our program using sys.argv, which gives a list of arguments. The first argument is always the name of the file (in our case, main.py). So any filename passed to our program would be the second argument:

https://github.com/nyjc-computing/pseudo/blob/0be6bb23111255a1e9c9fd9087e70169bab771b9/main.py#L15-L18

The result of running it in a shell might look like this:

$ python main.py main.pseudo
5 + 1 is 6

[ngjunsiang]

Now that's looking more respectable.

On to the next thing a proper compiler needs: error messages.