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IDProjectCategoryView StatusDate SubmittedLast Update
0005362OCamlOCaml generalpublic2011-09-27 11:032011-10-05 19:07
ReporterDmitry Grebeniuk 
Assigned To 
PrioritynormalSeverityfeatureReproducibilityalways
StatusacknowledgedResolutionopen 
PlatformOSOS Version
Product Version3.12.1 
Target VersionFixed in Version 
Summary0005362: "let _ : t = v;;" is not accepted without camlp4o
Description"let _ : t = v" should be accepted just like "let x : t = v" even without camlp4o:

$ ocaml
        Objective Caml version 3.12.1+rc1

# let x : int = 1;;
val x : int = 1
# let _ : int = 1;;
Error: Syntax error
# #use "topfind";;
- : unit = ()
Findlib has been successfully loaded. Additional directives:
  #require "package";; to load a package
  #list;; to list the available packages
  #camlp4o;; to load camlp4 (standard syntax)
  #camlp4r;; to load camlp4 (revised syntax)
  #predicates "p,q,...";; to set these predicates
  Topfind.reset();; to force that packages will be reloaded
  #thread;; to enable threads

- : unit = ()
# #camlp4o;;
/home/gds/dev/ovm/ocaml/lib/dynlink.cma: loaded
/home/gds/dev/ovm/ocaml/lib/camlp4: added to search path
/home/gds/dev/ovm/ocaml/lib/camlp4/camlp4o.cma: loaded
    Camlp4 Parsing version 3.12.1+rc1

# let x : int = 1;;
val x : int = 1
# let _ : int = 1;;
- : int = 1
#


Also it's strange that toplevel output ("- : int = 1") differs from revised syntax:


$ ocaml
        Objective Caml version 3.12.1+rc1

# #use "topfind";;;
- : unit = ()
Findlib has been successfully loaded. Additional directives:
  #require "package";; to load a package
  #list;; to list the available packages
  #camlp4o;; to load camlp4 (standard syntax)
  #camlp4r;; to load camlp4 (revised syntax)
  #predicates "p,q,...";; to set these predicates
  Topfind.reset();; to force that packages will be reloaded
  #thread;; to enable threads

- : unit = ()
# #camlp4r;;
/home/gds/dev/ovm/ocaml/lib/dynlink.cma: loaded
/home/gds/dev/ovm/ocaml/lib/camlp4: added to search path
/home/gds/dev/ovm/ocaml/lib/camlp4/camlp4r.cma: loaded
    Camlp4 Parsing version 3.12.1+rc1

# value _ : int = 1;
#


(there is no output at all)
TagsNo tags attached.
Attached Files

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-  Notes
(0006132)
garrigue (manager)
2011-09-27 12:21

 We have here a subtle point.
In the OCaml grammar, a let definition is either

  let <pattern> = <expression>

or

  let <ident> <args ...> [: <type>] = <expression>

In patterns, a type annotation requires parentheses, so

  let (_ : <type>) = <expression>

is of course ok, but

  let _ : <type> = <expression>

doesn't fit either.
The specification of the manual fits this description.
(0006133)
Dmitry Grebeniuk (reporter)
2011-09-27 16:15

 Yes, you are right.
So, please treat this issue not as "bug report", but as a "feature wish".
It's very convenient to think about "_" pattern as "/dev/null variable", "sink variable", "value that is will be bound to anything you'll bind it to". So, I personally like the #camlp4o and #camlp4r behaviour. However, I'm using revised syntax almost always, so this issue will cause inconvenience/misunderstanding only for "plain toplevel users".
(0006143)
Julien Signoles (reporter)
2011-10-04 09:51

 Usually I use "let __ : type = expr" (double underscore) when I want to use such a pattern.

- Issue History
Date Modified Username Field Change
2011-09-27 11:03 Dmitry Grebeniuk New Issue
2011-09-27 12:21 garrigue Note Added: 0006132
2011-09-27 16:15 Dmitry Grebeniuk Note Added: 0006133
2011-10-04 09:51 Julien Signoles Note Added: 0006143
2011-10-05 19:07 doligez Severity minor => feature
2011-10-05 19:07 doligez Status new => acknowledged

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