Re: [Caml-list] pattern matching on mapped lists

[Romain Bardou <romain.bardou@inria.fr>]

On 03/06/2015 00:23, Nils Becker wrote:
> I find this syntax handy
>
>      let [ii; jj] = List.map float_of_int [i; j] in
>      ... do stuff with the floats
>
> because it avoids repeating the function call. The compiler lets this
> pass but warns that the matching is not exhaustive since [] is not
> matched. This actually can't happen if I'm not mistaken. So, is this
> considered good style, or is there a better idiomatic way to do multiple
> assignments? If yes, should this be an enhancement request for the type
> checker?
>
> n.

The type-checker cannot find out that List.map returns a list of the 
same size. I think it would require dependent types. Maybe some GADT can 
encode this, but it would be a bit messy.

If you use this idiom often, maybe you should define some map functions 
on tuples, whose length is known at compile-time:

let map2 f (x, y) = f x, f y

let (ii, jj) = map2 float_of_int (i, j) in
...

Unfortunately you need to define map2, map3, map4...

Cheers,

-- 
Romain