[Caml-list] camlp4 'loc' variable

[Caml-list] camlp4 'loc' variable

[J. Garrett Morris]

Hello,

I'm currently writing a set of camlp4 quotations, and I'm running into the "Unbound value 'loc'" error that's been discussed on this list before.  I'm curious about exactly when the loc variable needs to be present, though.  I've got a fair number of helper routines like:

let processList (l:MLast.expr list):MLast.expr =
      List.fold_right (fun x l -> <:expr<[$x$ :: $l$]>>) l <:expr<[]>> 

which are used both in antiquotations and in let statements surrounding expr quotations ... when do I need to take 'loc' as a parameter?

Thanks.

Gary

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Re: [Caml-list] camlp4 'loc' variable

[Stefano Zacchiroli]

On Sun, Feb 22, 2004 at 01:06:47PM -0500, J. Garrett Morris wrote:
> which are used both in antiquotations and in let statements
> surrounding expr quotations ... when do I need to take 'loc' as a
> parameter?

The camlp4 manual states that the loc variable is bound in the "action"
part of a "level-rules" (i.e. the right hand side of an -> arrow in a
grammar production).

Therefore I guess you should pass loc as a parameter if you invoke
functions from those RHSs which are not defined inside a RHS.

Cheers.

-- 
Stefano Zacchiroli -*- Computer Science PhD student @ Uny Bologna, Italy
zack@{cs.unibo.it,debian.org,bononia.it} -%- http://www.bononia.it/zack/
If there's any real truth it's that the entire multidimensional infinity
of the Universe is almost certainly being run by a bunch of maniacs. -!-

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Re: [Caml-list] camlp4 'loc' variable

[Aleksey Nogin]

On 22.02.2004 10:06, J. Garrett Morris wrote:

> Hello,
> 
> I'm currently writing a set of camlp4 quotations, and I'm running into the "Unbound value 'loc'" error that's been discussed on this list before.  I'm curious about exactly when the loc variable needs to be present, though.  I've got a fair number of helper routines like:
> 
> let processList (l:MLast.expr list):MLast.expr =
>       List.fold_right (fun x l -> <:expr<[$x$ :: $l$]>>) l <:expr<[]>> 
> 
> which are used both in antiquotations and in let statements surrounding expr quotations ... when do I need to take 'loc' as a parameter?

Basically, whenever you have an <:expr< ... >> quotation used an 
expression, it will use the "loc" variable for the "location" part of 
the expression. For example, in your case

<:expr<[$x$ :: $l$]>>
   expands to
MLast.ExApp (loc, MLast.ExApp (loc, MLast.ExUid (loc, "::"), x), l)

and

<:expr<[]>>
   expands to
MLast.ExUid (loc, "[]")

You can find out how camlp4 expands things by running, for example, 
"camlp4 q_MLast.cmo pa_o.cmo pr_o.cmo your_file.ml" - the output will 
contain the "plain" ML code.

Hope this helps.

-- 
Aleksey Nogin

Home Page: http://nogin.org/
E-Mail: nogin@cs.caltech.edu (office), aleksey@nogin.org (personal)
Office: Jorgensen 70, tel: (626) 395-2907

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