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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: best practice: error handling
Date: Sat, 11 Jun 2011 15:48:38 +0200
Organization: Ada @ Home
Message-ID: <op.vwwzbcxiule2fv@douda-yannick>
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Date: 2011-06-11T15:48:38+02:00
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Le Tue, 31 May 2011 20:08:38 +0200, Jeffrey Carter  =

<spam.jrcarter.not@spam.not.acm.org> a =C3=A9crit:
> The correct way to handle this is for the function to return a variant=
  =

> record:
>
> type Index_Result (Found : Boolean :=3D False) is record
>     case Found is
>     when False =3D>
>        null;
>     when True =3D>
>        Index : Positive;
>     end case;
> end record;

That's an idiom I also use with SML (I feel to remember there is a  =

dedicated name for that idiom, but I forget what it is this name), as  =

access to the result is automatically constrained to whether or not the =
 =

function succeed. But while this is good for request-like function (like=
  =

get the item with this or that property), this come to be less handy whe=
n  =

many and many functions or procedure return a common type may also retur=
n  =

error, because you end to as many type definition.

Another case is the if-previous-succeed-then-go-on-else-bypass idiom.  =

Multiple procedure get an in/out boolen parameter. If on input, the  =

parameter is False, then the procedure bypass its own process, if the  =

input parameter is True, the procedure do its job and set the parameter =
to  =

False on output if ever it fails somewhere. This is a useful idiom with =
 =

SPARK which does not allow exceptions. Combined with that strategy, a  =

wrapper type would lead to a mess.

-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D  [Ep=
igrams on  =

Programming =E2=80=94 Alan J. =E2=80=94 P. Yale University]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [Idem]
=E2=80=9Cc++; /* this makes c bigger but returns the old value */=E2=80=9D=
 [Anonymous]