Re: [Email-SIG] invertability and idempotence

[Barry Warsaw]

On Oct 21, 2009, at 6:58 PM, R. David Murray wrote:

But it turns out that idempotence does have a meaning in the context
of the email module, so I think I need to remove 'depreciated' from
my glossary[1] entry for it, and explain what it means in the context
of the email module.

I think you're onto something here.

For background, see issue 7119[2].

Here's what I propose: _invertability_ applies to the data path
into the parser and out of the generator.  That is:

   generate(parse(msg)) == msg

should be true whenever possible.

Agreed, where 'msg' in this context means the message text or bytes.

On the other hand, when _constructing_ a message, sometimes not all data

is filled in (in the example above, it is the MIME boundary marker).

again should produce the _same_ output.  That is:

   generate(msg) == generate(msg)

In that case, it is important (I think, please discuss :) that generating the message maintain _idempotency_: once you have generated the message, then if you have not further mutated the message, generating the message even though the state of msg may change after the _first_ generate call.

-Barry

"Idempotent" means: "multiple applications of the operation do not change the result". So here where the operation is to take a message object and generate a stream of text or bytes, this should absolutely return the same stream if the object is not mutated between calls. I think it's fair though that if the model is manipulated in any way, we make no guarantees of idempotency, though we should strive for minimal differences.

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