[m-dev.] user-defined operators

[Fergus Henderson]

On 09-Jul-1999, Peter Ross <petdr at cs.mu.OZ.AU> wrote:
> 
> Would the haskell approach taken from the haskell-98 report be an
> acceptable compromise?
> 
> Alphanumeric operators are formed by enclosing an identifier between
> grave accents (backquotes). Any variable or constructor may
> be used as an operator in this way. If fun is an identifier (either
> variable or constructor), then an expression of the form fun x y is
> equivalent to x `fun`y. If no fixity declaration is given for `fun` then
> it defaults to highest precedence and left associativity
> 
> http::/www.cs.mu.oz.au/~fjh/haskell/haskell98-report-html/exps.html#sect3.2

I think this is an excellent suggestion.  This would be very simple to
implement, and would add very little complexity to language.  It would give
you much of the benefit of user-defined operators while having basically none
of the drawbacks.

The main drawbacks of user-defined operators are
	- they interact badly with the module system
	- they're difficult to implement
	- they can potentially make it difficult for human readers to
	  parse the code

In contrast, using backquotes for infix operators with a fixed precedence
and associativity has no bad interactions with the module system,
is very easy to implement, and since the precedence and associativity
are fixed, it should be pretty easy for human readers to parse.

This is simple enough and useful enough that I think it would be worth having
even if we were to go on and allow Prolog-style user-defined operators.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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