Re: [Caml-list] Re: '_a

[Keith Wansbrough <Keith.Wansbrough@cl.cam.ac.uk>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> > (b) the codomain is mis-specified, we actually have
> > 	List.hd: 'a list -> Some 'a
> 
> Funny, I always assumed that the domain of List.hd was "'a list - []".

Yes, indeed.  Were List.hd of type 'a list -> 'a option, we'd be stuck
if we ever wanted actually to _use_ the value: any function that
attempted to extract it (say f : 'a option -> 'a) would have to have a
similar type (f : 'a option -> 'a option).

The same problem would ensue if we had explicit exception typing:

List.hd : 'a list -> 'a with_possible_exception

extract : 'a with_possible_exception -> 'a option

really_extract : 'a option -> 'a with_possible_exception

and so on...!

The only way out of this mess is to add a monad.  OCaml already has
one: return is implicit, bind is called ";", and the monad operations
include "raise" and "try ... with ...".

HTH.

--KW 8-)