Publish a post about the new REPL

content/posts/valeri_repl_became_useful/note.md

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+X-Date: 2024-09-01T23:26:26Z
+X-Note-Id: b5ba6437-5fe2-4d01-8dc5-3efe8587e728
+Subject: Valeri REPL became actually useful (and keeps state)
+X-Slug: valeri_repl_became_useful
+
+[Valeri](https://git.sr.ht/~knazarov/valeri) REPL has progressed to a state where it can
+run multiple sequential statements that can refer to each other's variable and function
+bindings. A few examples:
+
+```
+valeri> (fn square (x) (* x x))
+#<function square>
+valeri> (square 5)
+25
+
+valeri> (def four 4)
+4
+valeri> (fn add-four (x) (+ x four))
+#<function add-four>
+valeri> (add-four 5)
+9
+```
+
+And in addition to supporting primitive arithmetics, it also got support for arrays and dicts:
+
+```
+valeri> (def a [1 2 (+ 3 4)])
+[1 2 7]
+valeri> (get a 2)
+7
+
+(def d {"foo" 123 "bar" 456})
+{"bar" 456 "foo" 123}
+valeri> (get d "bar")
+456
+```
+
+Of course all of this is available when you run scripts from a file as well, like `./valeri example.vli`.
+Compared to what I had just a week ago, this is a lot of progress. Previous version of the REPL could only
+compile one-shot operations that didn't share any state with each other, and as such the usefulness was
+very limited.
+
+The standard library is very tiny at the moment, and this means that you can't write bigger programs. For
+example, there is no `map`, `fold`, pipelining (or "threading"), reductions and other higher-order constructs.
+Because Valeri is "pure" as opposed to Scheme, Common Lisp and Clojure which all offer mutation, writing
+more complex programs will be tedious.
+
+But before jumping into adding functionality to the standard library, I'd like to tackle two more things:
+
+- Macros, as in other regular Lisps
+- Effects, for proper I/O and interactions between processes
+
+The lack of macros in particular stands in the way of the standard library, because many useful constructs
+have to be either implemented through them or be supported directly by the compiler. And I don't want to
+bloat the compiler too much.
+
+Effects are needed to build gradually more and more complex applications (like text filtering or a web server).
+At this moment, the only available effect is `println` which is implemented as a hack. The real implementation
+will utilize ideas similar to [Roc language](https://www.roc-lang.org/), where any I/O yields a continuation.
+
+But for now, I'm quite pleased with the results.