Issue #11997 has been updated by Shyouhei Urabe.


This is difficult to implement in-core than it sounds.  String interpolation is tightly built into our parser; hard to extirpate.  A built-in for this feature would accordingly result in the same eval + File.read maneuver, to just translate into C.

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Feature #11997: A method to read a file with interpolations
https://bugs.ruby-lang.org/issues/11997#change-57530

* Author: Tsuyoshi Sawada
* Status: Feedback
* Priority: Normal
* Assignee: 
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I request a single method that reads a file while interpreting interpolated Ruby code in it (in the current context). For example, suppose I have a file `foo.txt`.

foo.txt

~~~
#{char * 20}
Hello, my name is #{name}.
#{char * 20}
~~~

I am not sure about what method name the proposed feature should have, and on which class it should be defined, but let's temporarily call it `File.eval`. Then I would like to do:

~~~ruby
char = "*"
name = "John Doe"
File.eval("foo.txt") # => "********************\nHello, my name is John Doe.\n********************\n"
~~~

The use case that I have in mind is to easily create template engines/preprocessors, or do an equivalent of what such software do on the fly within a script. There are tools like erb, SCSS, haml, etc., but those force a particular syntax, and they are full fledged software with their own parsing capability, which means it takes an extra time to parse, and the sortware may have their own bugs. I often feel that I do not want to adopt any of the syntax forced in those limited choice of template/preprocessing software, but do it in a freer way.

Using what we have currently, I can do it with:

~~~ruby
eval('"' + File.read("foo.txt") + '"')
~~~

but that looks cumbersome, and wish if Ruby had a C-level built-in capability for doing that. I believe that would encourage wide range of Ruby users to more freely create their own templating system.



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