In other modern languages, such as Haskell or Python, a similar declarative functionality is also present out of the box. For example, you can perform filtering in Python—it is built into the language, and you have a special function in Haskell to perform the same filtering. Also, the functional nature of Python and Haskell makes it easy to implement the same control structure as filtering by yourself. Both Haskell and Python support the notion of the lambda function and higher-order functions, so they can be used to implement declarative control structures.
In general, you can spot whether a language is declarative programming-friendly by looking at the capabilities it provides. Some of the features you can look for are anonymous functions, functions as first-class citizens, and custom operator specifications.
Anonymous lambda gives you a great advantage because you can pass functions to other functions inline, without first defining them. This is particularly useful when specifying control structures. A function expressed in this way is, first and foremost, to specify a transformation that is supposed to transform an input into an output.
Another feature that you can look for in programming languages is support for functions as first-class citizens. This means that you are able to assign a function to a variable, refer to the function by that variable's name, and pass that variable to other functions. Treating functions as if they are ordinary variables allows you to achieve a new level of abstraction. This is because functions are transformations; they map their input values to some output values. And, if the language does not allow you to pass transformations to other transformations, this is a limitation of flexibility.
Another feature that you can expect from declarative languages is that they allow you to create custom operators; for example, the synthetic sugar available in Scala allows you to define new operators very easily, as methods in classes.
