Re: RFC: String interpolation

[Joost Kremers <joostkremers@fastmail.fm>]

On Thu, Dec 08 2016, Clément Pit--Claudel wrote:
> Hi all,
>
> Many languages are gaining literal string interpolations 
> features of late (the latest example is Python; see 
> https://www.python.org/dev/peps/pep-0498/).  The TL;DR is that 
> in Python 3.6 one can write this:
>
>     value = 1.3
>     print(f'The value is {value + 1:.2f}.')
>
> This prints "The value is 2.30."  Plenty of other languages have 
> similar features.
>
> It would be nice if Emacs offered something similar out of the 
> box. It could be a reader macro, but a plain macro would do just 
> as well, and indeed it's relatively easy to code something up (I 
> attached an example implementation).

That's definitely something that would be great to have. I'm not 
sure I would have chosen the same syntax, though.

> With the attached fmt.el, you can write things like this:
>
>     (let ((a 12) (b 15))
>       (fmt "$a + $b = $(+ a b)"))
>     ⇒ "12 + 15 = 27"

That syntax looks great to me, and I don't really see any reason 
to make it much more complex.

>     (let ((password "letmein"))
>       (fmt "The password is ${password}"))
>     ⇒ "The password is letmein"

Why the {}? Is there any context in which $<expr> is ambiguous 
such that ${<expr>} is needed to resolve it? Other than vectors, I 
mean, since you use [] for an optional format spec? Personally, 
I'd just say that vectors need to be written as (vector ...). 
[]-vectors are constants anyway, so in such cases you might as 
well write the string directly.

>     (let ((n 25) (price-per-unit 10.5))
>       (fmt "The price for $n units is €$[%.2f](* n 
>       price-per-unit)"))
>     ⇒ "The price for 25 units is €262.50"
> 
>     (fmt "Here's a symbol: '${'a}; "
>          "and a padded number: ‘$[%-6.3f]{1}’.")
>     ⇒ "Here's a symbol: 'a; and a padded number: ‘1.000 ’."
>
>     (fmt "Welcome to Emacs! "
>          "Press $[where-is]{'help-with-tutorial} to open the 
>          tutorial.")
>     ⇒ "Welcome to Emacs! Press C-h t to open the tutorial."
>
>     (fmt "Today is $[date|%Y-%m-%d](current-time).")
>     ⇒ "Today is 2016-12-07."

I honestly must say I find these last two very confusing. I would 
simply write:

(fmt "Today is $(format-time-string \"%Y-%m-%d\" current-time)).")

The escaped double quotes are not perfect, but I find this much 
more readable and more pleasant to look at.

Perhaps you should also look at abo-abo's hydra package, since he 
does basically the same thing in his hydra doc strings. AFAICT, it 
works this way: % starts a placeholder, ` marks the start of the 
expression. If the expression is an s-expr, you can leave out the 
`. So your examples would be:


>     (let ((a 12) (b 15))
>       (fmt "$a + $b = $(+ a b)"))

  (fmt "%`a + %`b = %(+ a b)")

>     (let ((password "letmein"))
>       (fmt "The password is ${password}"))

  (fmt "The password is %`password")

>     (let ((n 25) (price-per-unit 10.5))
>       (fmt "The price for $n units is €$[%.2f](* n 
>       price-per-unit)"))

(fmt "The price for %`n units is €%.2f(* n price-per-unit)") 

>     (fmt "Here's a symbol: '${'a}; "
>          "and a padded number: ‘$[%-6.3f]{1}’.")

  (fmt "Here's a symbol: '%`'a; "
       "and a padded number: ‘%-6.3f`1’.")

(Though here you might as well forego fmt altogether).

>     (fmt "Welcome to Emacs! "
>          "Press $[where-is]{'help-with-tutorial} to open the 
>          tutorial.")

  (fmt "Welcome to Emacs! "
       "Press %(fmt-whereis 'help-with-tutorial) to open the 
       tutorial.")

This one would require `fmt-whereis' to be an alias for your 
`fmt--printers-whereis'.

>     (fmt "Today is $[date|%Y-%m-%d](current-time).")

  (fmt "Today is %(format-time-string \"%Y-%m-%d\" 
  (current-time)).")

One addition might be to let %n stand for the value of the symbol 
n, and only interpret what immediately follows % as a format spec 
when there is also a backtick. So

  (fmt "The value of x is: %x")

would refer to the value of the variable s, while

  (fmt "The value of x is: %x`x")

would format the value of x in hexadecimal.

So, summarized:

- %<expr> : format the value of <expr> where <expr> can be a 
  symbol or an s-expr, possibly also a vector or a string 
  (somewhat superfluously...)

- %<fspec>`<expr> : format <expr> according to <fspec>.

Of course, It might be easier to use a different symbol for %, I 
don't know. I have nothing against using $.

> fmt expands the strings at compile time (so they must be 
> constant strings).  

I've occasionally appreciated the fact that I can pass a 
dynamically created format string to `format'... ;-) Ok, I could 
always use `format' in such cases.


-- 
Joost Kremers
Life has its moments