Ruby format string with variable inside quotes

Question

If I do

template_string = "class=btn submit-button %<additional_classes>"
format(template_string, additional_classes: 'some-class')

it works. However, if I do

template_string = "class='btn submit-button %<additional_classes>'"
format(template_string, additional_classes: 'some-class')

it fails, giving

ArgumentError:
  malformed format string - %'

(Notice the quotation marks surrounding the classes in the second template_string - this is the only difference between the two blocks of Ruby code). How do I make it work? In other words, how do I produce the following?

class='btn submit-button some-class'

I don't believe that I can just use interpolation, because sometimes I need to pass in other variables. In other words, I can't do

additional_classes = 'some-class'
"class='btn submit-button #{additional_classes}'"

because sometimes I want to reuse the same string "template" but pass in other variables, to produce strings such as the following:

class='btn submit-button some-other-class'

or

class='btn submit-button some-third-class'

mu is too short · Accepted Answer · 2018-08-29 22:31:41Z

2

From the fine manual:

format(format_string [, arguments...] ) → string
[...]
For more complex formatting, Ruby supports a reference by name. %<name>s style uses format style, but %{name} style doesn't.

The documentation isn't as clear as it could be but when you use the %<...> form, it is expecting to see %<name>s where name is the hash key and s is the format type: s for string, d for number, ... If you say:

%<additional_classes>'

then format will try to interpret ' as a type when there is no such type specifier so you get an ArgumentError because the format string is malformed.

You probably want to use the %{...} form instead:

template_string = "class='btn submit-button %{additional_classes}'"
#--------------------------------------------^------------------^

answered Aug 29, 2018 at 22:31

mu is too short

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Comments

Jörg W Mittag · Accepted Answer · 2018-08-29 22:35:46Z

Your format string is missing the field type specifier. The field type specifier is mandatory in a format string.

It is not clear to me why the first example does not raise an error, since the mandatory field type specifier is missing. It could be a bug, or I am completely misreading the documentation.

However, it is not clear to me why you consider this example to work:

template_string = "class=btn submit-button %<additional_classes>"
format(template_string, additional_classes: 'some-class')
#=> 'class=btn submit-button %'
#                            ↑

As you can see, the % is not interpreted as the part of a format string but as a literal %. I would consider this a bug, it should raise an error, just like the second example does.

In the second example, you can clearly see the problem:

ArgumentError: malformed format string - %'
                                          ↑

Since a format string must have a field type specifier, and the only character after the % (except the field name) is ', this is interpreted as the field type specifier. And since ' is not a legal field type, format raises an error in which it explicitly tells you that it interpreted the ' as part of the format string.

Since you want to format strings, you should use the s (string) field type specifier:

template_string = "class=btn 'submit-button %<additional_classes>s'"
#                                                                ↑

format(template_string, additional_classes: 'some-class')
#=> "class=btn 'submit-button some-class'"
#                             ↑↑↑↑↑↑↑↑↑↑↑

Alternatively, you can use the %{} form:

template_string = "class=btn 'submit-button %{additional_classes}'"
#                                            ↑                  ↑

format(template_string, additional_classes: 'some-class')
#=> "class=btn 'submit-button some-class'"
#                             ↑↑↑↑↑↑↑↑↑↑↑

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