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"Can someone explain to me why Haskell will ever be viable as a fullstack language?"

Aside from the solid compiler ghc and many libraries, the ability to write less, more reliable software. That's the objective isn't it?

Another reason is the increasing number of usable languages written in Haskell. Using say Purescript, elm or other front-end languages, it may be as important as knowing Haskell as know C is for Python, for example.