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RE: Functional composition operator?
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Date: -- (:)
From: Don Syme <dsyme@m...>
Subject: RE: Functional composition operator?
> 
>  -- it only save a few characters in programs
>      (Compare
>       let h = f o g 
>       with
>       let h x = f (g x);;)

As you say, you can always define the infix identifier, so there's no
hassle, i.e. I'm not complaining.  However, the comparison should be
    f1 << f2 << f3 << f4 
v.
    (fun x -> f1(f2(f3(f4 x))))

because we may not be binding the on-the-fly value to a name.  Saving
parentheses is clearly a good idea - most the ML syntax is directed to this.
Even worse consider

    f1 << f2 x y z << f3 << f4 a b

v.
   (let f4' = f4 a b in
    let f2' = f2 x y z in
    (fun x -> f1(f2' z(f3(f4' x)))))

since in the latter case we may be taking advantage of the partial
evaluation of the closures to, for example, precompute some tables based on
the information in x,y,z and a,b.  I know which I prefer!

In 16,000 lines implementing the interactive theorem prover DECLARE there
are 220 uses of composition, so one every 70 lines or so.  There are 500
uses of "map", so it's relatively common in that one random example at
least.

Cheers,
Don

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