[Caml-list] Feature Wish: List comprehensions

[Caml-list] Feature Wish: List comprehensions

[Jonathan Roewen]

Hi,

I'm having a small problem trying to tidily define an equivalent
function to some Haskell code.

Haskell:
pThen combine p1 p2 toks
  = [ (combine v1 v2, toks2) | (v1,toks1) <- p1 toks, (v2, toks2) <- p2 toks1 ]

(I'll also be needing to specify equivalents for pThen3 & pThen4).

The first isn't too bad...

let pThen combine p1 p2 = fun ts ->
  let l1 = p1 ts in
  let l2 = List.map (fun (v1,ts1) -> p2 ts1) l1 in
  List.concat
    (List.map2 (fun (v1,ts1) l2 -> List.map (fun (v2,ts2) -> combine
v1 v2,ts2) l2) l1 l2)

As you can see, extending this style to pThen3 & pThen4 is going to be
very, very ugly (and hard to get right first time).

I know it's just shorthand for map/filter, (and more complex
derivatives of these), but it would be nice to have in ocaml...

BTW: any suggestions on a better way to write the above function would
be appreciated ;-)

Kindest Regards,

Jonathan Roewen

Re: [Caml-list] Feature Wish: List comprehensions

[Martin Jambon]

There's a camlp4 lib for this:
   http://oandrieu.nerim.net/ocaml/#pa_compr

If you use ocamlfind, you might want install it with p4ck, which also 
installs a few other camlp4 hacks from various authors.
(http://martin.jambon.free.fr/p4ck.html)

Martin

On Thu, 30 Mar 2006, Jonathan Roewen wrote:

> Hi,
>
> I'm having a small problem trying to tidily define an equivalent
> function to some Haskell code.
>
> Haskell:
> pThen combine p1 p2 toks
>  = [ (combine v1 v2, toks2) | (v1,toks1) <- p1 toks, (v2, toks2) <- p2 toks1 ]
>
> (I'll also be needing to specify equivalents for pThen3 & pThen4).
>
> The first isn't too bad...
>
> let pThen combine p1 p2 = fun ts ->
>  let l1 = p1 ts in
>  let l2 = List.map (fun (v1,ts1) -> p2 ts1) l1 in
>  List.concat
>    (List.map2 (fun (v1,ts1) l2 -> List.map (fun (v2,ts2) -> combine
> v1 v2,ts2) l2) l1 l2)
>
> As you can see, extending this style to pThen3 & pThen4 is going to be
> very, very ugly (and hard to get right first time).
>
> I know it's just shorthand for map/filter, (and more complex
> derivatives of these), but it would be nice to have in ocaml...
>
> BTW: any suggestions on a better way to write the above function would
> be appreciated ;-)
>
> Kindest Regards,
>
> Jonathan Roewen
>
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--
Martin Jambon, PhD
http://martin.jambon.free.fr

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