which languages have a REPL and Type Annotation? (lemmy.blahaj.zone)

submitted 2 months ago by jh29a@lemmy.blahaj.zone to c/programming_languages@programming.dev

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(as an interested university student:) From Programming at a small scale in Javascript and Logo¹, I have gathered that not knowing what type something is can be annoying. They also have REPLs, which is pretty nice. From various blog posts and debugging Rust programs, I have learned that not having a REPL can be annoying. Are there languages that have both?

(¹ Logo is a "lisp" with omitable parentheses, where these also don't define runtime-mutable s-expressions, lists are in brackets, and also Logo doesn't have structs, giving it bad maintainability outside of not having Type Annotation too)

Candidates

C# : Does it have a repl?
Java in BlueJ somehow
sometimes people just put Lisp or Lua in their C/Rust++ program (emacs, shenzhen I/O(game)), this accomplishes a similar task of making some debugging or scripting code faster to compile/interpret, but slower to run

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[-] Ephera@lemmy.ml 5 points 2 months ago

I guess, Python technically counts, although the type annotations were one hell of an afterthought.

I do feel like the two features are somewhat at odds with each other.
A REPL is useful for quickly trying something out, which you'd typically use to put together a quick script. In a larger application, you generally want to define unit tests, which make your "quickly trying something out" reproducible and automateable.
Meanwhile, type annotations primarily start to shine in larger applications, where you cannot keep the whole context in your head and where refactorings become problematic, when nothing checks that the parts still work together.

In some sense, they also don't work well together. Type annotations work best when the whole context is known. But in a REPL, you almost always have an incomplete context, because you're still typing everything out.
So, it cannot type-check the first few lines you write and it becomes awkward to try to show type errors once the context is complete, because the code isn't on-screen anymore.

this post was submitted on 26 Oct 2025

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