I think Haskell and OCaml are popular in finance because they're high-level and familiar to math/quant types (who are broadly--and this is a sweeping generalization--more familiar with expressing algorithms in recursion than in procedural sequences).

I don't have much experience with OCaml, but I can also say that Haskell in particular has incredible support for building very high-level libraries that are both expressive and have strong compile-time guarantees. I've found it's relatively easier for a programming expert to build such a library and a domain expert to consume the library in Haskell than in many other languages I've seen (perhaps Python comes the closest, but that feels to me more like an ecosystem thing than a language thing).

[1] https://serokell.io/blog/haskell-in-production-standard-char...

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