Allow files to be loaded from the REPL

[brendanzab]

We should be able to load files in the REPL, either when passing a list of filenames on startup, or by a :load/:l command while the REPL is running:

cargo run repl ./src/library/prelude.pi

Pikelet> :load ./src/library/prelude.pi
Pikelet> id String "hi"
"hi" : #String

We should also be able to reload files using :reload/:r:

Pikelet> :load ./src/library/prelude.pi
Pikelet> :reload

[brendanzab]

One thing that might complicate this is that we currently parse REPL commands as part of the main grammar:

pikelet/src/syntax/parse/grammar.lalrpop

Lines 58 to 87 in 770959b

	pub ReplCommand: ReplCommand = {
	=> ReplCommand::NoOp,
	<term: Term> => ReplCommand::Eval(Box::new(term)),
	<start: @L> <command: "REPL command"> <end: @R> =>? match command {
	"?" | "h" | "help" => Ok(ReplCommand::Help),
	"q" | "quit" => Ok(ReplCommand::Quit),
	command => {
	let span = ByteSpan::new(start, end);
	let command = String::from(command);
	Err(LalrpopError::User { error: ParseError::UnknownReplCommand { span, command} })
	},
	},
	<start: @L> <command: "REPL command"> <end: @R> <term: Term> =>? match command {
	"core" => Ok(ReplCommand::Core(Box::new(term))),
	"t" | "type" => Ok(ReplCommand::TypeOf(Box::new(term))),
	command => {
	let span = ByteSpan::new(start, end);
	let command = String::from(command);
	Err(LalrpopError::User { error: ParseError::UnknownReplCommand { span, command} })
	},
	},
	<start: @L> <command: "REPL command"> <end: @R> <ident: Ident> "=" <term: Term> =>? match command {
	"let" => Ok(ReplCommand::Let(ident, Box::new(term))),
	command => {
	let span = ByteSpan::new(start, end);
	let command = String::from(command);
	Err(LalrpopError::User { error: ParseError::UnknownReplCommand { span, command} })
	},
	},
	};

It will complicate things because we'll also need to include a 'file path' token in the lexer, which feels a little excessive. One way to hack around this until we find a better solution would be to use Token::String as the argument to :load.

Including the repl stuff in the main parser has always felt a tad unpleasant though, especially the way we match on the command names. Not sure of a better solution though...

[boomshroom]

Currently, load_file generates its own TcEnv and DesugarEnv. While TcEnv can be added to after the fact, DesugarEnv can only be expanded by generating new identifiers. In order for this to work, we'd need to be able to add any exports from the module to both the TcEnv and the DesugarEnv of the repl, but without including the repl's environment with it (though the prelude and any future imports should be included).

[brendanzab]

I'm wondering if load_file could take a TcEnv instead? 🤔