0004886: Desugaring of camlp4 quotations of labeled patterns duplicates variables

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Date Submitted

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0004886

OCaml

Camlp4

public

2009-10-03 20:31

2012-09-18 04:29

Reporter

yallop

Assigned To

garrigue

Priority

normal

Severity

minor

Reproducibility

always

Status

feedback

Resolution

open

Platform

OS Version

Product Version

3.11.1

Target Version

4.00.2+dev

Fixed in Version

Summary

0004886: Desugaring of camlp4 quotations of labeled patterns duplicates variables

Description

Here's a function that extracts the label from a typed labeled pattern:

  open Camlp4.PreCast

  let extract_label = function
    | <:expr< fun ~ ($lab$:$_$) -> $_$ >> -> lab

Here's what happens when you try to compile it:

  $ ocamlc -I +camlp4 -pp camlp4of -c patt.ml
  File "patt.ml", line 4, characters 11-14:
  Error: Variable lab is bound several times in this matching

This is surprising to the user because 'lab' only appears once in the pattern in the source file.

Here's the camlp4 output, showing that the variable 'lab' has been duplicated:

  $ camlp4of patt.ml
  open Camlp4.PreCast

  let extract_label =
    function
    | Ast.ExFun (_,
        (Ast.McArr (_,
           (Ast.PaLab (_, lab,
              (Ast.PaTyc (_, (Ast.PaId (_, (Ast.IdLid (_, lab)))), _)))),
           (Ast.ExNil _), _)))
        -> lab

Here's where I think the duplication occurs in the camlp4 source:

        | "~"; "("; i = a_LIDENT; ":"; t = ctyp; ")" ->
            <:patt< ~ $i$ : ($lid:i$ : $t$) >>

        (camlp4/Camlp4Parsers/Camlp4OCamlParser.ml)

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Notes
(0008101) garrigue (manager) 2012-09-18 04:29	I think there is no bug here, but a misunderstanding about label syntax: fun ~(lab:typ) -> exp is an abbreviated syntax equivalent to fun ~lab:(lab:typ) -> exp and they share the same abstract syntax representation. For this reason, if you want to get the label, you should just write: <:expr< fun ~$lab$:$_$ -> $_$ >> (Not checked)

Issue History
Date Modified	Username	Field	Change
2009-10-03 20:31	yallop	New Issue
2009-10-03 20:31	yallop	Status	new => assigned
2009-10-03 20:31	yallop	Assigned To	=> ertai
2012-07-11 13:34	doligez	Target Version	=> 4.01.0+dev
2012-07-12 11:29	doligez	Assigned To	ertai =>
2012-07-12 11:29	doligez	Status	assigned => acknowledged
2012-07-31 13:36	doligez	Target Version	4.01.0+dev => 4.00.1+dev
2012-09-17 17:37	doligez	Target Version	4.00.1+dev => 4.00.2+dev
2012-09-18 04:29	garrigue	Note Added: 0008101
2012-09-18 04:29	garrigue	Assigned To	=> garrigue
2012-09-18 04:29	garrigue	Status	acknowledged => feedback