Friday, 4 May 2012

Why are functions not recursive by default in F#?

The French and British descendants of the original ML made different choices and their choices have been inherited through the decades to the modern variants. So this is just legacy but it does affect idioms in these languages.
Functions are not recursive by default in the French CAML family of languages (including OCaml and F#). This choice makes it easy to supercede function (and variable) definitions using let in those languages because you can refer to the previous definition inside the body of a new definition. F# inherited this syntax from OCaml.
For example, superceding the function p when computing the Shannon entropy of a sequence in OCaml:
let shannon fold p =
  let p x = p x *. log(p x) /. log 2.0 in
  let p t x = t +. p x in
  -. fold p 0.0
Note how the argument p to the higher-order shannon function is superceded by another p in the first line of the body and then another p in the second line of the body.
Conversely, the British SML branch of the ML family of languages took the other choice and SML'sfun-bound functions are recursive by default. When most function definitions do not need access to previous bindings of their function name, this results in simpler code. However, superceded functions are made to use different names (f1f2 etc.) which pollutes the scope and makes it possible to accidentally invoke the wrong "version" of a function. And there is now a discrepancy between implicitly-recursive fun-bound functions and non-recursive val-bound functions.
Haskell makes it possible to infer the dependencies between definitions by restricting them to be pure. This makes toy samples look simpler but comes at a grave cost elsewhere.

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