LLMs use tokens, with 1d positions and rich complex fuzzy meanings, as their native "syntax", so for them LISP is alien and hard to process.
That's like reading binary for humans. 1s and 0s may be the simplest possible representation of information, but not the one your wet neural network recognizes.
Already over two years ago, using GPT4, I experimented with code generation using a relatively unknown dialect of Lisp for which there are few online materials or discussions. Yet, the results were good. The LLM slightly hallucinated between that dialect and Scheme and Common Lisp, but corrected itself when instructed clearly. When given a verbal description of a macro that is available in the dialect, it was able to refactor the code to take advantage of it.
Agreed, Gleam as a language has very few, generalized syntactic constructs compared to most procedural languages. There's enough of a signal in the data to be able to answer queries about the language; but when writing, LLMs universally trip over themselves. The signal from other nearby languages is too strong and it ends up trying to do early returns, if statements, even loops on occasion.
I usually use deepseek (gratis) for code, and when using defun and let it usually lacks one (or more) closing parenthesis. So the way to mark the end is not well understood by this LLM, or perhaps that the height of the AST is usually bigger than in python.
That's like reading binary for humans. 1s and 0s may be the simplest possible representation of information, but not the one your wet neural network recognizes.