repl

I just really wanted to make a repl. Hopefully I'll find a nice name for it later:)

🗣️ Shoutout Val

What can it do?

It's loosely based on R5RS: Legacy Scheme, the programming language with the prettiest syntax, objectively.

Here are some expressions:

repl, version 0.0.1

ь > 1
1

ь > '(1 2)
'(1 2)

ь > (+ 2 3 (* 1 (- 2 9) 9))
-58

Oh yeah, we use polish notation baby 😎

Is that it?

Far from it. Here are some statements

ь > (define currentYear 2025) -- I know ie the DrRacket repl doesn't return this. Sue me!
#<val:currentYear>

ь > (define lastYear 2024)
#<val:lastYear>

ь > (- currentYear lastYear)
1

ь > (define (plus x y) (+ x y))
#<procedure:plus>

ь > (define (plus1 x) (plus x 1))
#<procedure:plus1>

ь > (plus1 10)
11

Wait, what's up with ь?

Oh, that's just a мягкий знак (soft sign), my favourite letter in the Cyrillic alphabet. It "serves as an indicator of palatalization of the preceding consonant" [1].

What else?

For debugging I find it valuable to print my program's AST. After the REPL became perfect I removed it, but then I got thinking, why not keep it on as a feature? So if you wanna see your program's AST you can prefix your programs with ast

ь > ast 1
Expression (Constant (Number 1))

ь > ast x
Expression (Variable "x")

ь > ast (define x 1)
Statement (VariableDeclaration "x" (Number 1))

R5RS Scheme does not allow cadr nesting beyond 4 levels, but don't worry, I do

ь > (define lst '(1 2 3 4 5 6 7 8 9 10 11))
#<val:lst>

ь > (cadddddr lst)
6

also I'm bad at keeping track of parentheses so in my perfect little world I'll allow as many as the eye can see

ь > (cond ((#f 1) (#f 2) (#t 3)))))))
3

just like God intended

Hmm, let me try this

Go to your terminal and run

stack install

and then I think

stack build

and if that goes well you can do

stack run

and you should be good. Or else your guess is as good as mine 😄

I'm just assuming you have stack and parsec and all that installed 😇😘

Help, I'm stuck in the repl forever

Don't worry, you can actually leave whenever you want, you just have to say goodbye! Because an Irish goodbye would hurt its (my) feelings 😇

repl, version 0.0.1

ь > exit()
Eval error: "Variable not found: exit"

ь > :quit
Parse error: (line 1, column 1):
unexpected ":"
expecting "(", identifier, literal string, float, integer, "#t", "#f" or "'"

ь > ciao
Arrivederci caro 👋
sergey@Sergeys-MBP repl %

This means you cannot have variables/procedures called ciao.. which is a well thought-out feature of course...

References

[1] Wikipedia, Wikimedia Foundation, 16 May 2025. Soft sign. https://en.wikipedia.org/wiki/Soft_sign