I've spoken a little about the reason we couldn't use GHC among other things, at https://github.com/replit/repl.it/issues/43. In short, we need an interpreter rather than a compiler or a JIT, and we need the interpreter itself to run in the browser.

Thanks for a quick reply. So having GHCi - the GHC's REPL - successfully emscripted could make adding Haskell support possible, right?

If it uses a JIT internally, it will produce x86 machine code, which is meaningless to us since we aren't emulating the whole machine. If it doesn't, then emscripting it should work.

Haskell won't run in a repl, will it? GHCi and Hugs demand that functions be specified in a script file and not at the prompt.

Without functions, you really don't have very much of the Haskell language available. It's a partial repl with a fraction of the language at best.

Not necessarily. You can define functions in GHCi using let.

  $ ghci
  Prelude> let fact x = if x == 0 then 1 else x * fact (x - 1)
  Prelude> fact 6
  720