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Custom operators in the revised syntax
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Date: -- (:)
From: Jon Harrop <jon@f...>
Subject: Re: [Caml-list] Custom operators in the revised syntax
On Saturday 12 May 2007 10:49, you wrote:
> What your system should do when encountering:
> let double x = x + x;;

F# ossifies global definitions to defaults (if any) or type errors. In this 

  val double : int -> int

If the definition had been:

  let first a = a.[0];;

then there is no default and it would ask for clarification as to the type 
of "a" (is it an array, hash table, vector etc.).

> double 1;;


> double 1.1;;
> double (Vector.from_array [| 1.1; 2.2; 3.3 |]);;

Both static type errors.

This seems to work very well by minimizing confusion, maximizing performance 
(everything is resolved statically) and letting me write mathematical 
expressions concisely.

If the definition of "double" is nested then its type is not ossified 
immediately but will unify to "int" at the application "double 1;;", so the 
next two calls are static type errors just as they are in OCaml.

There are other problems, like do we generalize from:

  val ( * ) : 'a -> 'a -> 'a


  val ( * ) : 'a -> 'b -> 'c

e.g. "scalar * vector -> vector"?

You also have ambiguity with conventional syntax. Does "vector * vector" mean 
dot, cross or element-wise product?

Lessons can be learned from languages like Matlab, where people often 
accidentally add a scalar to each element of a vector (so we know that would 
be a useful type error).

Potentially, you could even overload "f x" to allow it to mean multiplication 
when "f" is a numeric and not a functional value. So the type could go 
through ['a -> 'b | int | float | ...].

I've written about 100k LOC of F# code now and I haven't had a single 
non-trivial type problem caused by overloading. So I'd definitely question 
the conventional view that overloading and inference do not sit well 
together. Worst case, the compiler asks for a type annotation...

Dr Jon D Harrop, Flying Frog Consultancy Ltd.
The F#.NET Journal