Re: Tangling flow control

["Charles C. Berry" <ccberry@ucsd.edu>]

On Wed, 9 Nov 2016, Philip Hudson wrote:

> On 9 November 2016 at 17:54, Charles C. Berry <ccberry@ucsd.edu> wrote:
>> On Tue, 8 Nov 2016, Philip Hudson wrote:
>>
>> [snip]
>>>
>>> How do you do "looping" flow control?
>>>
>>> For context, what I'm trying to write is a single Org file from which
>>> I can tangle out a number of =~/.ssh/config= files, one for each of
>>> several hosts on a LAN. Within this file I need to repeatedly place a
>>> template =BEGIN_SRC ssh-config= block, each time with a few words and
>>> numbers changed. Do you do this anywhere? If so, how have you
>>> implemented it?
>>
>> It sounds like what you want is a template for the src block and another src
>> block that does substitutions in that template using a table of values
>> inside a loop.
>>
>> Just to get you started, with this template:
>>
>> #+NAME: template
>> #+BEGIN_SRC org
>>   ,#+BEGIN_SRC shell :tangle %to-file
>>   ls -lt %filename
>>   ,#+END_SRC
>> #+END_SRC
>>
>> and this helper src-block
>>
>> #+NAME: get-body
>> #+BEGIN_SRC emacs-lisp :var src-block-name="c-code"
>>   (save-excursion
>>     (org-babel-goto-named-src-block
>>      src-block-name)
>>     (cadr (org-babel-get-src-block-info)))
>>
>> #+END_SRC
>>
>>
>> running
>>
>> #+header: :wrap src org :var tmpl=get-body("template")
>> #+BEGIN_SRC emacs-lisp
>>   (org-fill-template tmpl
>>     '(("to-file" . "abc.sh")("filename" . "my-dir")))
>> #+END_SRC
>>
>> yields
>>
>> #+RESULTS:
>> #+BEGIN_src org
>> ,#+BEGIN_SRC shell :tangle abc.sh
>> ls -lt my-dir
>> ,#+END_SRC
>> #+END_src
>>
>>
>> To revise this for your application, you need to provide a table of the
>> associated values for the "to-file" and "filename" keys in the alist, read
>> that table using a :var header, loop thru the table reconstructing the alist
>> each time creating src blocks, and send the output to an org tempfile.  Then
>> you tangle the tempfile.
>>
>> Alternatively, you simply write the script files directly without bothering
>> to write to an org tempfile.
>
> Thanks Chuck. I think I've got that straight. I hadn't come across
> `org-fill-template' before. I think I know how to loop thru a table,
> though I haven't done it myself before; I've seen how to "get" a table
> as a list.
>
> What I'm not clear about is why the template nests a shell-script
> block inside an Org block. Should the outer Org block not have a
> %-escaped placeholder for a :tangle target, an intermediate Org file?

No.

The org src block is just a container. Its body is a src block template 
that gets copied into the variable `tmpl', which if filled and placed in 
file can be tangled.

Alternatively, you can eval (setq tmpl "<template string>") somewhere 
(maybe in a local variables block), where the <template string> part is 
the template.

But it seems easier to edit the template as an org src block - C-c ' puts 
your template in an OrgSrc buffer, then moving point to a src block and 
typing M-x org-edit-src-code puts its body in another OrgSrc buffer where 
you can edit it in the shell-mode or whatever mode.

Also, I suppose that template should have been an ssh-config src block to 
conform with your earlier posting. But I think you get the idea.

Chuck