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The substitution can be formatted using a syntax that looks like method calls


>= 4.2, < 7
~> 3.2
~> 1.8
 Project Readme


Handles parsing an input string with embedded substitution parameters and replacing them with values from a provided mapping.
The substitution can be formatted using a syntax that looks like method calls. For example, the following will return "it is a WINDY day".

ParameterSubstitution.evaluate( input: "it is a <weather.upper> day", mapping: { "weather" => "windy" } )


Add this line to your application's Gemfile:

gem 'parameter_substitution'

And then execute:

$ bundle

Or install it yourself as:

$ gem install parameter_substitution


ParameterSubstitution is configured by registering a custom_formatters hash:

ParameterSubstitution.configure do |config|
  config.custom_formatters = { 'example_formatter' => 'ExampleFormatter' }

These formatters are specified in your app. They must inherit from ParameterSubstitution::Formatters::Base and must implement description and format methods.

So, in the "it is a WINDY day" example at the beginning of this README, the call to upper formats the input by calling Upper.format.

class Upper < ParameterSubstitution::Formatters::Base
  def self.description
    "Converts to string and returns all characters uppercased, preserves nil"

  def self.format(value)
    value && value.to_s.upcase


Formats can take arguments and can be chained. For example, the following formats the time using strftime to find just am or pm, and then compares that to say morning or evening.

  input:   "good <time.date_time_strftime("%p").compare_string("am", "morning", "evening")>",
  mapping: { "time" => }

The substitution behavior is very configurable because it is used in many different environments. The configuration is passed through named arguments to the evaluate method. These named arguments have defaults that should work for most conditions.

Default Formatters

In addition to the custom formatters that you can specify in your app (see Configuration), ParameterSubstitution has some default formatters that are available for use. Below is a list of some of the most common ones. You can see all available formatters in formatters.

Formatter Description
add_prefix This takes a prefix as a constructor parameter and prepends it to the value. If the value is blank, nothing is shown
cgi_unescape Url-decodes the string, preserves nil
compare_string Compares the field to a string and returns results based on the comparison
date_time_strftime Formats a DateTime with the provided format string
downcase Converts to string and downcases the values, preserves nil
greater_than_value Compares numerical values and returns results based on the comparison
if_nil Takes one new_value parameter. If the input is nil, the input is replaced with new_value
if_truthy If the input is truthy (i.e. true, "t", 1, "on", "yes") then the input is replaced with the first argument. Otherwise, the input is replaced with the second argument
left Takes a single n argument. Returns the left most n characters from the input
lower Converts to string and returns all characters lowercased, preserves nil
right Takes a single n argument. Returns the right most n characters from the input
trim Returns the input as a string with leading and trailing whitespace removed
upper Converts to string and returns all characters uppercased, preserves nil



The ParameterSubstitution module exposes the public interface for this subsystem. All other classes are internal and should be considered private.

When evaluate is called, the module constructs a Parser to parse the input into a syntax tree. Parser is implemented using the Parslet gem, which is pretty great. The output from the parser is a ruby hash that describes the input.

The module then constructs a Transform to convert this syntax tree to an Expression that can be evaluated.
An Expression has a list of sub expressions that are either TextExpressions, which simply returns the text when evaluated, or SubstitutionExpressions which return the formatted value from the mapping when called.

SubstitutionExpressions have a list of MethodCallExpressions. These expressions call the corresponding class in 'app/models/reporting/column_formats'

Context is a helper class that is used to pass around the set of options for the current parameter substitution.

Next Steps

If you are working in this subsystem, consider the following improvements while you work:

  • The performance is pretty slow. I believe the performance is suffering because it is building a parser for every request. Parsers take a couple input parameters, but really there are just a couple permutations. (What type of open and close characters, are unbalanced closes allowed.) We could have a parser factory that kept the parsers that were constructed for each permutation.
  • The error handling for missing methods could be better. We raise the first error instead of all of the errors, and we could provide the location for each validation failure.
  • It would be nice if the column format methods provided a validation for their arguments.
  • The behavior for TokenReplacement.substitute_params is very odd. Missing substitution parameters and nil subsitution parameters are handled differently for keys vs values and if the subtitution is the full string or part of the string. I don't think this behavior is desired by customers and a more consistent handling would make for much cleaner code. (See any code that is handling the :raw destination format)

Running tests...

bundle exec rake test


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at[USERNAME]/parameter_substitution.