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I don't know about Haskell specifically, but the question isn't "how many developers are able and willing to work with $language?" but "how many developers are able and willing to work with $language relative to the number of companies hiring for it?".

For example, Elm is still a tiny, niche language, but from what I hear it's relatively easy to hire for Elm because there are many more people who want to work with it than there are companies that are hiring for it.