pg quote from the article: you don’t just write your program down toward the language, you also build the language up toward your program. As you’re writing a program you may think "I wish Lisp had such-and-such an operator." So you go and write it. Afterward you realize that using the new operator would simplify the design of another part of the program, and so on.

I remember thinking when I first read through pg's essays a few years back and again now that you can do this in declarative languages as well using functions.

For example, my programs often looks something like this:

  main(){
    var data = getData();
    pushData(data);
  }

  function getData(){
    var data = fetchDataFromSource();
    var errors = validateData(data);
    if(errors != null)
      //handle errors
    return processData(data);
  }

  function fetchDataFromSource(){
    ...
  }

  function validateData(var data){
    ...
  }

  ...

Could anyone explain why functional languages are better for this? It seems to me this is the same as "building the language up to your program".