In OCaml, the value restriction leads to non-generalized type variables ('_a etc.) if you try to define functions like:
# let ( << ) f g x = f(g x);; val ( << ) : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = <fun> # let cons h t = h::t;; val cons : 'a -> 'a list -> 'a list = <fun> # cons << cons;; - : '_a -> ('_a list -> '_a list) list -> ('_a list -> '_a list) list = <fun> This is a silly example but you are most likely to hit this problem in practice in the context of parser combinators. Due to JIT compilation, F# cannot relax the value restriction so that does not even compile. In MLs, you usually want the eta-expanded form: # let cons2 x = (cons << cons) x;; val cons2 : 'a -> ('a list -> 'a list) list -> ('a list -> 'a list) list = <fun> But a pipeline is prettier: # let ( |> ) x f = f x;; val ( |> ) : 'a -> ('a -> 'b) -> 'b = <fun> # let cons2 x = x |> cons |> cons;; val cons2 : 'a -> ('a list -> 'a list) list -> ('a list -> 'a list) list = <fun> This is one advantage of Haskell over OCaml/F#. However, I don't see it as a useful advantage in practice because parser combinators are so tedious during development (they require constant attention as types evolve): you want code generation like ocamlyacc or camlp4. OCaml is a very strong contender here, of course. Cheers, Jon. > -----Original Message----- > From: m...@proof-technologies.com [mailto:m...@proof-technologies.com] > Sent: 10 November 2010 13:44 > To: jonathandeanhar...@googlemail.com; ymin...@gmail.com; > ar...@noblesamurai.com > Cc: caml-l...@inria.fr > Subject: Re: [Caml-list] Infix function composition operator > > So how does value restriction affect things here? (excuse my lack of > knowledge) > > One thing about using a pipeline like this is that it relies on '|>' > being > left-associative (which it is due to OCaml's convention on operators > that > start with "|"). > > Mark. > > > on 10/11/10 12:52 PM, Jon Harrop <jonathandeanhar...@googlemail.com> > wrote: > > > A pipeline operator is usually preferred over function composition in > impure > > languages like OCaml and F# due to the value restriction. For > example, > your > > example would be written in F# as: > > > > x |> op1 |> op2 |> op3 |> op4 |> op5 > > > > This style is very common in F#, particularly when dealing with > collections. > > > > Cheers, > > Jon. > > > >> -----Original Message----- > >> From: caml-list-boun...@yquem.inria.fr [mailto:caml-list- > >> boun...@yquem.inria.fr] On Behalf Of m...@proof-technologies.com > >> Sent: 10 November 2010 07:00 > >> To: ymin...@gmail.com; ar...@noblesamurai.com > >> Cc: caml-l...@inria.fr > >> Subject: Re: [Caml-list] Infix function composition operator > >> > >> on 10/11/10 3:45 AM, ymin...@gmail.com wrote: > >> > >> > This is probably a minority opinion, but I have written and read > >> quite a > >> lot > >> > of OCaml code over the years, and I've seen surprisingly few > >> effective > >> uses > >> > of the composition operator. Somehow, I usually find that code > that > >> avoids > >> > it is simpler and easier to read. > >> > >> I agree that using a composition operator can make the code obtuse, > and > >> so > >> should not be overused. But it's incredibly useful for certain > >> situations: > >> > >> 1) If you are performing a long chain of composed operations, it > avoids > >> nested bracketing piling up. > >> > >> For example: > >> (op5 <<- op4 <<- op3 <<- op2 <<- op1) x > >> Instead of: > >> op5 (op4 (op3 (op2 (op1 x)))) > >> > >> This sort of thing happens quite a lot in certain applications, e.g. > in > >> language processing, to get at subexpressions. > >> > >> 2) Creating an anonymous function to be passed as an argument, it > >> avoids > >> explicitly mentioning arguments of that function. > >> > >> This sort of thing can happen a lot in functional programming > >> generally. > >> > >> For example: > >> List.map (op2 <<- op1) xs > >> Instead of: > >> List.map (fun x -> op2 (op1 x)) xs > >> > >> Mark Adams > >> > >> _______________________________________________ > >> Caml-list mailing list. Subscription management: > >> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list > >> Archives: http://caml.inria.fr > >> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners > >> Bug reports: http://caml.inria.fr/bin/caml-bugs > > > > > > > > _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs