Why can't the Mail block see my variable?

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生来不讨喜
生来不讨喜 2021-02-15 11:35

I\'m new to Ruby and wondering why I am getting an error in this situation using the \'mail\' gem in a simple Sinatra app:

post \"/email/send\" do

  @recipient          


        
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  • 2021-02-15 11:42

    As Julik says, Mail#delivery executes your block using #instance_exec, which simply changes self while running a block (you wouldn't be able to call methods #to and #from inside the block otherwise).

    What you really can do here is to use a fact that blocks are closures. Which means that it "remembers" all the local variables around it.

    recipient = params[:email]
    Mail.deliver do 
        to recipient # 'recipient' is a local variable, not a method, not an instance variable
    ...
    end
    

    Again, briefly:

    • instance variables and method calls depend on self
    • #instance_exec changes the self;
    • local variables don't depend on self and are remembered by blocks because blocks are closures.
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  • 2021-02-15 11:44

    I think it's because the Mail gem uses instance_exec under the hood. instance_exec uses instance variables from the object it's being called on and not from the caller. What I'd do is find a method in the Mail gem that does not use instance tricks but passes an explicit configuration object to the block, and proceed from there. Spares a few gray hairs.

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  • 2021-02-15 12:03

    If you'll read through the docs for Mail further you'll find a nice alternate solution that will work. Rather than use:

    Mail.deliver do 
      to @recipient # throws error as this is undefined
      from 'server@domain.com'
      subject 'testing sendmail'
      body 'testing sendmail'
    end
    

    you can use Mail's new() method, passing in parameters, and ignore the block:

    Mail.new(
      to:      @recipient,
      from:    'server@domain.com',
      subject: 'testing sendmail',
      body:    'testing sendmail'
    ).deliver!
    

    or the alternate hash element definitions:

    Mail.new(
      :to      => @recipient,
      :from    => 'server@domain.com',
      :subject => 'testing sendmail',
      :body    => 'testing sendmail'
    ).deliver!
    

    In pry, or irb you'd see:

    pry(main)> Mail.new(
    pry(main)* to: 'me@domain.com',
    pry(main)* from: 'me@' << `hostname`.strip,
    pry(main)* subject: 'test mail gem',
    pry(main)* body: 'this is only a test'
    pry(main)* ).deliver!
    => #<Mail::Message:59273220, Multipart: false, Headers: <Date: Fri, 28 Oct 2011 09:01:14 -0700>, <From: me@myhost.domain.com>, <To: me@domain.com>, <Message-ID: <4eaad1cab65ce_579b2e8e6c42976d@myhost.domain.com>>, <Subject: test mail gem>, <Mime-Version: 1.0>, <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>
    

    The new method has several variations you can use. This is from the docs also, and might work better:

    As a side note, you can also create a new email through creating a Mail::Message object directly and then passing in values via string, symbol or direct method calls. See Mail::Message for more information.

     mail = Mail.new
     mail.to = 'mikel@test.lindsaar.net'
     mail[:from] = 'bob@test.lindsaar.net'
     mail['subject'] = 'This is an email'
     mail.body = 'This is the body'
    

    followed by mail.deliver!.

    Also note, in the previous example, that there are multiple ways to access the various headers in the message envelope. It's a flexible gem that seems to be well thought out and nicely follows the Ruby way.

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