Why are records not 'export'-able? - Erlang Programming

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Message from discussion Why are records not 'export'-able?
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Date: Fri, 29 Jun 2012 16:27:10 +0100 (BST)
From: Robert Virding <robert.vird...@erlang-solutions.com>
To: Mahesh Paolini-Subramanya <mah...@dieswaytoofast.com>
Message-ID: <495b5f26-3eee-4263-b9dd-d1d6456dc111@knuth>
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Subject: Re: [erlang-questions] Why are records not 'export'-able?
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The main reason is that records are purely compile-time and record definitions only exist in the compiler. At runtime a record is just a normal tuple with nothing in it to show that it came from a record or what the record definition is. So there are no problems accessing record as tuples and tuples as records, though in most cases it is a stupid thing to do. 

So the problem is that you need to be able to access the record definition whenever a module using it is compiled. One solution is to put the record definition .hrl file in an applications include directory. It is then possible to access that file from the compiler using: 

-include_lib("app/include/recdef.hrl"). 

The compiler will extend the app name with version numbers when necessary. 

Robert 

----- Original Message -----

> If I have a record I use in just one module, I declare it in that
> module.
> If I have a record I use across a number of modules, I declare it in
> an included file.
> If I have a record that I use across a number of applications, I end
> up declaring it once in each application.

> The simple-minded me says "Hmf. If I could just have a
> '-export_record' declaration, then I could do some fun things like
> '-record(Name, Application:Record)'

> The more realistic me says "There is probably some reason why you
> can't do this. Probably has to do with compilers. It usually does".

> Anyone?

> Mahesh Paolini-Subramanya
> That Tall Bald Indian Guy...
> Blog | Twitter | Google+

> _______________________________________________
> erlang-questions mailing list
> erlang-questi...@erlang.org
> http://erlang.org/mailman/listinfo/erlang-questions

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>The main reason is that records are purely compile-time and record definit=
ions only exist in the compiler. At runtime a record is just a normal tuple=
 with nothing in it to show that it came from a record or what the record d=
efinition is. So there are no problems accessing record as tuples and tuple=
s as records, though in most cases it is a stupid thing to do.<br><br>So th=
e problem is that you need to be able to access the record definition whene=
ver a module using it is compiled. One solution is to put the record defini=
tion .hrl file in an applications include directory. It is then possible to=
 access that file from the compiler using:<br><br>-include_lib("app/include=
/recdef.hrl").<br><br>The compiler will extend the app name with version nu=
mbers when necessary.<br><br>Robert<br><br><hr id=3D"zwchr"><blockquote sty=
le=3D"border-left:2px solid rgb(16, 16, 255);margin-left:5px;padding-left:5=
px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;fon=
t-family:Helvetica,Arial,sans-serif;font-size:12pt;">If I have a record I u=
se in just one module, I declare it in that module.<div>If I have a record =
I use across a number of modules, I declare it in an included&nbsp;file.</d=
iv><div>If I have a record that I use across a number of applications, I en=
d up declaring it once in each application.</div><div><br></div><div>The si=
mple-minded me says "Hmf. If I could just have a '-export_record' declarati=
on, then I could do some fun things like '-record(Name, Application:Record)=
'</div><div><br></div><div>The more realistic me says "There is probably so=
me reason why you can't do this. Probably has to do with compilers. It usua=
lly does".</div><div><br></div><div>Anyone?</div><div><br><div>
<span class=3D"Apple-style-span" style=3D"border-collapse: separate; color:=
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ite-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-=
spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decoration=
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eparate; font-variant: normal; letter-spacing: normal; line-height: normal;=
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ne; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horiz=
ontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-deco=
rations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-strok=
e-width: 0px; "><div style=3D"word-wrap: break-word; -webkit-nbsp-mode: spa=
ce; -webkit-line-break: after-white-space; "><div><div><span class=3D"Apple=
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n class=3D"Apple-style-span" style=3D"border-collapse: separate; font-varia=
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rmal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px;=
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none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><di=
v><div style=3D"font-family: Helvetica; margin-top: 0in; margin-right: 0in;=
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an" color=3D"#1f497d" face=3D"Calibri, sans-serif"><span class=3D"Apple-sty=
le-span" style=3D"font-size: 15px; "><div style=3D"margin-top: 0px; margin-=
right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal norma=
l 15px/normal Calibri; color: rgb(31, 73, 125); "><div style=3D"margin-top:=
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x; font: normal normal normal 15px/normal Calibri; color: rgb(1, 108, 226);=
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px; margin-left: 0px; font: normal normal normal 15px/normal Calibri; color=
: rgb(1, 108, 226); "><a href=3D"http://www.gravatar.com/avatar/204a87f81a0=
d9764c1f3364f53e8facf.png" target=3D"_blank"><b><i>Mahesh Paolini-Subramany=
a</i></b></a></div><div style=3D"margin-top: 0px; margin-right: 0px; margin=
-bottom: 0px; margin-left: 0px; font: normal normal normal 15px/normal Cali=
bri; color: rgb(31, 73, 125); ">That Tall Bald Indian Guy...</div><div styl=
e=3D"margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0=
px; font: normal normal normal 15px/normal Calibri; color: rgb(1, 108, 226)=
; "><span style=3D"text-decoration: underline; "><a href=3D"http://dieswayt=
oofast.blogspot.com/" target=3D"_blank">Blog</a></span><span class=3D"Apple=
-converted-space">&nbsp;</span>&nbsp;<span class=3D"Apple-converted-space">=
&nbsp;</span><span style=3D"color: rgb(31, 73, 125); ">|<span class=3D"Appl=
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nk"><span style=3D"text-decoration: underline; color: rgb(1, 108, 226); ">T=
witter</span></a><span class=3D"Apple-converted-space">&nbsp;</span>&nbsp;<=
span class=3D"Apple-converted-space">&nbsp;</span>|<span class=3D"Apple-con=
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" target=3D"_blank"><span style=3D"text-decoration: underline; color: rgb(1=
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pan></font></div></div></span></span></div></div></div></span></div></span>=
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ttom: 0px; margin-left: 0px; font: normal normal normal 15px/normal Calibri=
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an></div></span></div></span></div></span></div></span></div></span></span>=

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