why do some languages require function to be declared in code before calling?

Question

Suppose you have this pseudo-code

do_something();

function do_something(){
   print "I am saying hello.";
}

Why do some programming languages require the call to do_something() to appear below the function declaration in order for the code to run?

Answer 1

Programming languages use a symbol table to hold the various classes, functions, etc. that are used in the source code. Some languages compile in a single pass, whereby the symbols are pulled out of the symbol table as soon as they are used. Others use two passes, where the first pass is used to populate the table, and then the second is used to find the entries.

Answer 2

Most languages with a static type system are designed to require definition before use, which means there must be some sort of declaration of a function before the call so that the call can be checked (e.g., is the function getting the right number and types of arguments). This sort of design helps both a person and a compiler reading the program: everything you see has already been defined. The ease of reading and the popularity of one-pass compilers may explain the popularity of this design rule.

Unfortunately definition before use does not play well with mutual recursion, and so language designers resorted to an ugly hack whereby you have

1. Declaration (sometimes called a "forward declaration" from the keyword in Pascal)
2. Use
3. Definition

You see the same phenomenon at the type level in C in the form of the "incomplete struct declaration."

Around 1990 some language designers figured out that the one-pass compiler with no abstract-syntax tree should be a thing of the past, and two very nice designs from that era—Modula-3 and Haskell got rid of definition before use: in those languages, any defined function or variable is visible throughout its scope, including parts of the program textually before the definition. In other words, mutual recursion is the default for both types and functions. Good on them, I say—these languages have no ugly and unnecessary forward declarations.

Why [have definition before use]?

• Easy to write a one-pass compiler in 1975.

• without definition before use, you have to think harder about mutual recursion, especially mutually recursive type definitions.

• Some people think it makes it easier for a person to read the code.