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Date: -- (:)
From: skaller <skaller@u...>
Subject: Re: [Caml-list] best and fastest way to read lines from a file?
On Tue, 2007-10-02 at 20:02 +0200, kirillkh wrote:
> Replying to a private mail from Brian:

> (* I couldn't figure out, how to declare a polymorphic exception
> properly *)
> exception Done of 'a

That's easy -- you can't: even if you could, how could
you possibly use it?

This compiles fine:

type t = { field : 'a. 'a }
exception Done of t

but 'field' is useless. This is not at all the same as

	let f (x:'a) (g:'a -> int) =
	match g x with
	| 0 -> ..
	| ..

because *inside* the function, 'a is not a type variable,
and the code is not polymorphic, it is simply a sole 
unknown type, sometimes said to be monomorphised.

The problem with exceptions is that they're not captured,
so they cannot be polymorphic. Exceptions SUCK because
their context is not delimited -- you can throw all the way
out of the mainline .. :)

[This happens to me regularly and it can takes days to figure
out what is Not_found ..]

-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net