Haskell would be my vote, and Rust too, actually, both because of their very strong type systems. The type system lets you very quickly figure out what something is before you figure out what something does, and it turns out that separating those two concerns as hard as those two languages do often results in doing the whole one-two punch faster.
Haskell does not qualify for a large training set, though. (Nor for readability in my opinion)
I think I have never seen haskell software made wih LLM's but well, aside from university, I have not seen Haskell code at all. (Also Haskell purists I would associate with people who avoid LLM's)
I would rather go with Rust given these choices.
But I have good results with typescript (or javascript for simpler things). Really large set of examples. Tools optimized for it, agents debugging in the browser works allmost out of the box. And well, a elaborate typesystem.
I used Claude to generate Haskell and it works really well. Claude struggles sometimes with respecting abstraction boundaries, but Haskell enforces parts of those boundaries in its type system better than a lot of other languages (if a module can’t do IO, for example).
Works well, in my experience. Sometimes the agent does weird stuff that you have to rewrite, but I get the sense that this happens in any language.
Maybe Haskell’s training set is not large enough, but it seems to work despite the smaller training set.
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Bit of a nit, it isn't the strong typing that makes Rust great for LLMs, it's the very strict compiler.
Plenty of languages have strong (enough) typing but their compilers happily let you or the LLM footgun yourself.