How can I export definitions of several symbols to a single file?

Question

the problem related to this question

What I want to do is to Export definition of several symbol into a single file. The reason I choose Export is that Export supports the GZIP function.

So here is the example: I have

ff[1, 1] = 1;
ff[1, 2] = 2;
gg[1, 1] = 3;
gg[1, 2] = 4;

if I use Save, then it is simply

Save["ffgg.m", {ff, gg}]

the content of ffgg.m file

ff[1, 1] = 1
ff[1, 2] = 2
gg[1, 1] = 3
gg[1, 2] = 4

But if I want to Export, since the syntax of Export is Export["file", expr]. It seems that I can only Export one expr each time. I tried the following code:

Export["ffgg2.m", {FullDefinition @ ff, FullDefinition @ gg}];

the content of ffgg2.m will be

(* Created by Wolfram Mathematica 8.0 : www.wolfram.com *)
{ff[1, 1] = 1
ff[1, 2] = 2, gg[1, 1] = 3
gg[1, 2] = 4}

And if I Import ffgg2.m back, I only get ff[1,1] and gg[1,1]. The other two lost.

So how can I Export several definitions into a single file at a single time?

Answer 1

FullDefinition

You can use the undocumented(?) multi-argument syntax of FullDefinition:

ExportString[FullDefinition[ff, gg], "Package"]

(* Created by Wolfram Mathematica 7.0 : www.wolfram.com *)

ff[1, 1] = 1

ff[1, 2] = 2

gg[1, 1] = 3

gg[1, 2] = 4

You will notice that any arguments of FullDefinition after the first will be colored, by default in red. This usually signifies a syntax error, but more specifically here it means the syntax doesn't match the pattern given in SyntaxInformation. There are a number of functions that work with syntax somewhat different form that which is officially supported (documented). Of course such functionality may change or be removed in later versions so you have to weigh the alternatives. If you are comfortable with this caveat you can eliminate the red highlighting with:

Unprotect[FullDefinition]

SyntaxInformation[FullDefinition] = {"ArgumentsPattern" -> {__}}

Protect[FullDefinition]

---

The OP asked:

I searched some questions relate Undocumented functions. Many people suggest "don't use it". So is there any other way to realize the same thing?

Yes, but none that I know of that is as concise.

PutAppend

You can use PutAppend to append definitions to a file:

Cases[Hold[ff, gg], x_ :> (FullDefinition@x >>> "file.m")];

(* contents of file *)
ff[1, 1] = 1

ff[1, 2] = 2
gg[1, 1] = 3

gg[1, 2] = 4

This uses multiple "export" (or rather PutAppend) commands, but it seems to do what you want.

Even more manually you can ExportString each definition then concatenate them:

strings = Cases[Hold[ff, gg], x_ :> ExportString[FullDefinition@x, "Package"]];

Export["file.m", StringJoin @ strings, "Text"]  (* note "Text" format *)