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DSSL2:   Data Structures Student Language
7.4

DSSL2: Data Structures Student Language

Jesse A. Tov <jesse@cs.northwestern.edu>

 #lang dssl2 package: dssl2

DSSL2 is a programming language for data structures students. It’s designed to be simple, providing only the basic elements needed to implement a variety of data structures.

DSSL2 uses alignment and indentation to delimit blocks. In particular, compound statements such as if-elif-else take ⟨block⟩s for each condition, where a ⟨block⟩ can be either one simple statement followed by a newline, or a sequence of statements on subsequent lines that are all indented by four additional spaces. Here is an example of a tree insertion function written using indentation:

#lang dssl2
 
def insert!(t, k):
    if empty?(t): new_node(k)
    elif zero?(random(size(t) + 1)):
        root_insert!(t, k)
    elif k < t.key:
        t.left = insert!(t.left, k)
        fix_size!(t)
        t
    elif k > t.key:
        t.right = insert!(t.right, k)
        fix_size!(t)
        t
    else: t

Each block follows a colon and newline, and is indented 4 spaces more than the previous line. In particular, the block started by def is indented by 4 spaces, and the elif blocks by 8. When a block is a simple statement, it can be placed on the same line, as in the if and else cases.

Extraneous indentation is an error.