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In Haskell in 5 steps the factorial function is defined as follows:
let fac n = if n == 0 then 1 else n * fac (n-1)
But for hugs, it says that fac needs to be in fac.h. Can anyone explain why this is the case – missing the ability to define named functions seems like a massive limitation for an interpreter?
The basic answer as far as I can tell is that the Hugs interactive toplevel is essentially an expression parser, and function/data definitions are not expressions. Your example actually would work if you made it an expression and wrote
let fac n = if n == 0 then 1 else n * fac (n-1) in fac 19. Adding support for this would be a pretty big effort, and apparently the Hugs implementors thought that it was good enough to require function/data definitions to be in files.