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Pattern matching in let defintions
f#compiler

Hi,

While fiddling around I stumbled across a syntax I didnt expect to work:

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  let r0 = List.nth r 0
  let r1 = List.nth r 1
  let r2 = List.nth r 2
  let r3 = List.nth r 3

can be written as:

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  let r0::r1::r2::r3::_ = r  
  let [r0;r1;r2;r3] = r

With the caveat that the compiler issues a warning about an incomplete pattern match. Is there anyway of avoiding the warning, or providing some form of default value or exception to the let binding which is provided upon failure?

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  let [r0;r1;r2;r3] = r | raise My_error
  let [r0;r1;r2;r3] = r | [0;0;0;0]

Cheers,

Andy

.

Indeed, what appears to be a variable name on the left of a let is actually a pattern. So you can deconstruct a tuple, for example:

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let a, b = 3, 4

or even nested tuples:

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let a, (b, c) = 3, (4, 5)

In your case, given that you're taking the first four elements off a list you should use a pattern match with a catch-all for the eventuality of a list with <4 elements:

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let r0, r1, r2, r3 = match list with
  | r0::r1::r2::r3::_ -> r0, r1, r2, r3
  | _ -> invalid_arg "My list"

However, you may want to replace your use of a list with a 4-tuple but I can't tell without more context.

Cheers,
Jon.

By on 11/30/2006 2:28 PM ()

Thanks you your comments chaps.

I realize this can all be done using a match expression. I was thinking that since this is essentially a pattern match, is there, or could there be, a special catch all clause in let bindings. It's all a case of reducing typing (but also avoiding all warnings). Without it, at least for lists (which I use alot because I like recursion), I'd write the List.nth version simply because with a bit of copy&paste its the shortest to type if not the most elegant.

Cheers,

Andy

By on 11/30/2006 4:04 PM ()

And do you know, is there a way to do something like that:

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let rec myFunction (hA::tA) (hB::tB) = hA+hB + (myFunction tA tB)

This example works, but I want to have a pattern for empty list, and I can't find information about whis construction.

I tried to use function keyword:

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let rec pair =
    function | ((f:float->float->float),(hx::tx:float list),(hw::tw:float list),acc) -> pair (f,tx,tw,(acc + (f hx hw)))
             | ((f:float->float->float),[],[],acc) -> acc
             | ((f:float->float->float),_,[],acc) -> failwith "Lists length are not equal!"
             | ((f:float->float->float),[],_,acc) -> failwith "Lists length are not equal!"

But here I am to use a tuple, that doesn't suits me.

By on 10/2/2008 2:39 AM ()

Hi,

You can use the match keyword, instead.

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let rec pair f a b acc =
  match a, b with
  | [], [] -> acc
  | ha::ta, hb::tb -> pair f ta tb (acc + f ha hb)
  | _ -> failwith "Lists length are not equal!"

If I understand correctly, you can also use library functions:

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let pair f a b = List.map2 f a b |> List.sum

Laurent

By on 10/2/2008 4:57 AM ()

Thank you for your reply!

I know about match keyword, but this Prolog-style function definition seems to me more elegant.

I liked your solution of your problem very much! :)

By on 10/2/2008 7:48 AM ()

Maybe :

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let [r0;r1;r2;r3] = 
    match r with 
    | a::b::c::d::e::_ ->  a::b::c::d::e::[]
    | _ -> 0::0::0::0::0::[]
By on 11/29/2006 11:19 PM ()

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