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Using Icon lists as stacks

Lists don't have to stay the same size. You can add or subtract elements at either end of a list. This allows you to use a list as a pushdown stack.

• The operation ``push(L, x)'' adds x as the new first element of list L.
• The operation ``pop(L)'' removes the first element of list L and returns its value; this function fails if the list is empty.

Here is an example. This program writes the lines of its input in reverse order, by pushing each line as it is read, and then writing each line as it is popped:

procedure main()
  stack := [];    # Make `stack' an empty list

  while push ( stack, read ( ) );   # Read and push each line
  while write ( pop ( stack ) );    # Pop and write each line
end