6 Differences Between Iterate and Loop

loop contains a great deal of complexity which iterate tries to avoid. Hence many esoteric features of loop don't exist in iterate. Other features have been carried over, but in a cleaned-up form. And of course, many new features have been added; they are not mentioned in this list.

• iterate's syntax is more Lisp-like than loop's, having a higher density of parens.
• The current implementation of iterate, unlike the current version of loop (as documented in Common Lisp, 2nd Ed.), is extensible (see Rolling Your Own).
• loop puts the updates of all driver variables at the top of the loop; iterate leaves them where the driver clauses appear.
• While for the most part iterate clauses that resemble loop clauses behave similarly, there are some differences. For instance, there is no for... =... then in iterate; instead use for... initially... then.
• loop binds the variable it at certain times to allow pseudo-English expressions like when expr return it. In iterate, you must bind expr to a variable yourself. Note that when expr return it is like thereis expr except that the latter is an accumulation clause and therefore competes with other accumulations (remember “Multiple Accumulations” in Other Features).
• loop has a special return clause, illustrated in the previous item. iterate doesn't need one, since an ordinary Lisp return has the same effect.
• loop allows for parallel binding and stepping of iteration variables. iterate does not. (See Parallel Binding and Stepping.)
• loop and iterate handle variable type declarations very differently. loop provides a special syntax for declaring variable types, and does not examine declarations. Moreover, the standard implementation of loop will generate declarations when none are requested. iterate parses standard Common Lisp type declarations, and will never declare a variable itself unless declarations are specifically requested.