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CHANGELOG.md (298, 2023-09-07)
LICENSE (1071, 2023-09-07)
Makefile (1009, 2023-09-07)
example/ (0, 2023-09-07)
example/Main.hs (1751, 2023-09-07)
flake.lock (2046, 2023-09-07)
flake.nix (2326, 2023-09-07)
package.yaml (1395, 2023-09-07)
snail-files/ (0, 2023-09-07)
snail-files/basic.snail (341, 2023-09-07)
snail-files/fail-comment.snail (7, 2023-09-07)
snail-files/fail-empty.snail (0, 2023-09-07)
snail-files/fail-nil.snail (12, 2023-09-07)
snail-files/fail-quotes-2.snail (19, 2023-09-07)
snail-files/fail-quotes-3.snail (13, 2023-09-07)
snail-files/fail-quotes.snail (24, 2023-09-07)
snail-files/fail.snail (52, 2023-09-07)
snail-files/nil.snail (4, 2023-09-07)
snail.cabal (3175, 2023-09-07)
src/ (0, 2023-09-07)
src/Snail.hs (159, 2023-09-07)
src/Snail/ (0, 2023-09-07)
src/Snail/Ast.hs (1441, 2023-09-07)
src/Snail/Characters.hs (573, 2023-09-07)
src/Snail/IO.hs (749, 2023-09-07)
src/Snail/Lexer.hs (3588, 2023-09-07)
src/Snail/ToText.hs (632, 2023-09-07)
test/ (0, 2023-09-07)
test/Gen.hs (330, 2023-09-07)
test/Snail/ (0, 2023-09-07)
test/Snail/IOSpec.hs (2125, 2023-09-07)
test/Snail/LexerSpec.hs (6193, 2023-09-07)
test/Snail/ToTextSpec.hs (962, 2023-09-07)
test/Spec.hs (208, 2023-09-07)
test/files/ (0, 2023-09-07)
test/files/fennel-reference.fnl (4824, 2023-09-07)
test/files/learnxyz.cl (3256, 2023-09-07)
test/files/r5rs_pitfalls.scm (10355, 2023-09-07)
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# Snail A no-semantics programming language for gastropods. ## Why? My colleagues and I are going to start working through [Types and Programming Languages][tapl]. In the book you implement languages of varying feature sets. The book implements these languages in OCaml, however I had this Lisp parser essentially ready for awhile. There are a handful of "Write you a Scheme Interpreters"-like tutorials and they all use a parser relatively similar to this one. However, there are some pretty subtle issues with most of the ones I have seen. For example, the two examples below parse as two lexemes in a lot of examples. Even Haskell's parser has [this issue][haskell-parse-issue]! ``` (1a) (1 a) ``` ## Is this really a programming language? From the ["Programming language" Wikipedia page][pl-wikipedia], > A programming language is a system of notation for writing computer programs. > The description of a programming language is usually split into the two components of syntax (form) and semantics (meaning) Snail is used for writing interpreters or compilers. However, it doesn't define **any** semantics. So, maybe? ## Syntax (form) Snail describes valid lexemes, text literals, and s-expressions. The valid lexemes are approximately from R5RS Scheme but this may change in the future. We also use Haskell's line and block comments. Here is a valid snail program, ``` -- Prints `hello "world"` to the console (print "hello \"world\"") -- Prints 3 to the console (print (+ 1 2)) {- Defines a function to add two numbers Applies the function to generate 3 Prints 3 to the console -} (let (f (lambda (x y) (+ x y))) (print (f 2 1))) (quote hello) (nil) (print true) (print false) -- end comment ``` Reminder, this program has no semantics. It is your job to take Snail's Abstract Syntax Tree (AST) and define the semantics of an interpreter or compiler. ## Getting the AST You have two options: `readSnailFile` or `parseSnail`. `readSnailFile` can be used like this, assuming you have put some valid snail into a file `./hello.snail`, ```haskell import Snail printSnail :: IO () printSnail = do eResults <- readSnailFile "./hello.snail" case eResults of Right ast -> print ast Left failureString -> print failureString ``` `parseSnail` doesn't require `IO` the only parameter is `Text`. This is useful for one-line programs, e.g., ```haskell {-# LANGUAGE OverloadedStrings #-} import Snail example :: Either String [SnailAst] example = parseSnail "(print false)" ``` ## Example Interpreters 1. The `arith` language from [Types and Programming Languages][tapl]: https://github.com/chiroptical/snail-arith/blob/main/src/Lib.hs 2. Languages from [essentials-of-compilation][essentials-of-compilation]: https://github.com/chiroptical/essentials-of-compilation (each chapter is a module) ## Differences from S-Cargot I was recently reminded of [s-cargot][s-cargot]. From their GitHub, > S-Cargot is a library for parsing and emitting S-expressions, designed to be > flexible, customizable, and extensible. This is quite different from Snail. Snail is not flexible, customizable, or extensible. I have chosen a syntax and either you use it or you don't. With S-Cargot, you can build a custom S-expression parser. Snail provides one. I don't think you can re-write Snail using S-Cargot? It doesn't appear to support `'(x)` or `(let [x 10] x)`. Please correct me if I missed something. I tried to mimic their example (https://github.com/aisamanra/s-cargot/blob/master/example/example.hs) in ./example/Main.hs. ## Resources for implementing interpreters and compilers These are resources I am personally interested in exploring, there are **many** others - [Types and Programming Languages][tapl] - [Essentials of Compilation][essentials-of-compilation] - [Compiler Design][cs4410] - [Compiler Construction][cse131] [tapl]: https://www.cis.upenn.edu/~bcpierce/tapl [haskell-parse-issue]: https://twitter.com/chiroptical/status/1471568781906518018 [pl-wikipedia]: https://en.wikipedia.org/wiki/Programming_language [essentials-of-compilation]: https://mitpress.mit.edu/9780262047760/essentials-of-compilation [s-cargot]: https://github.com/aisamanra/s-cargot [cs4410]: https://course.ccs.neu.edu/cs4410sp21/ [cse131]: https://ucsd-cse131-f19.github.io/

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