0005625: Exception aliasing not available in signatures - MantisBT

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ID

Project

Category

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Date Submitted

Last Update

0005625

OCaml

OCaml general

public

2012-05-29 05:27

2012-06-01 05:29

Reporter

thelema

Assigned To

garrigue

Priority

normal

Severity

feature

Reproducibility

N/A

Status

resolved

Resolution

no change required

Platform

OS

OS Version

Product Version

3.12.1

Target Version

Fixed in Version

Summary

0005625: Exception aliasing not available in signatures

Description

An exception can be defined as equal to another exception:
exception-definition ::= exception constr-name = constr
But within a .mli file, one cannot expose this equality:
specification ::= exception constr-decl

Steps To Reproduce

add:

exception Empty = Stack.Empty

to a .mli file

Additional Information

Batteries has use for "exception Empty = Stack.Empty" in our batStack.mli file, otherwise we'll have a BatStack.Empty and a Stack.Empty that aren't the same. We could remove the exception from the .mli file, but then it wouldn't be in the documentation as it should, and the test that (BatStack : module type of Stack) would fail, and would need... hmm, I don't think that the exception can be removed from the module type. But that's another issue.

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Notes
(0007468) garrigue (manager) 2012-05-29 07:55	There is a confusion here. An exception constructor is just a value, that you can re-export at the value level using the "exception E = <path>" construct. There is no need to show the sharing in the interface: they are identical at the value level.

(0007470) thelema (reporter) 2012-05-29 15:48	Ah, my mistake. I was testing using (==), but this is apparently a bad function to use on exceptions. # module A = struct exception X end;; module A : sig exception X end # module B : sig exception X end = struct exception X = A.X end;; module B : sig exception X end # B.X = A.X;; - : bool = true # B.X == A.X;; - : bool = false # A.X == A.X;; - : bool = false Thanks for the quick clarification.

(0007490) frisch (developer) 2012-06-01 05:29	A new exception value is created each time you refer to the exception constructor, so you get physically different values (and == returns false). "=" is also not very good on exceptions, as it compares only the name of the constructors (as strings) and, if they match, the arguments (even if they have different types). If you can, you should prefer pattern matching.

Issue History
Date Modified	Username	Field	Change
2012-05-29 05:27	thelema	New Issue
2012-05-29 07:55	garrigue	Note Added: 0007468
2012-05-29 07:55	garrigue	Status	new => resolved
2012-05-29 07:55	garrigue	Resolution	open => no change required
2012-05-29 07:55	garrigue	Assigned To	=> garrigue
2012-05-29 15:48	thelema	Note Added: 0007470
2012-06-01 05:29	frisch	Note Added: 0007490

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