Non-local Exits and Loops

Non-local exits (catch and throw) and loops (loop and do) have been provided for two reasons:

1. to make the integration of existing COMMON LISP programs into REL programs easier
2. to allow tail-recursive deterministic REL relations and functions to be translated into LL loops in order to speed up execution

For a detailed explanation of catch, throw, loop, and do, see [\protect\citeauthoryearSteele Jr.1984]. Here a short description:

• (catch tag . body)
• (throw tag expr): leave the next catch body tagged with tag, returning expr
• (loop . body): repeat executing body; returns only if (throw nil expr) is executed (this is slightly different from COMMON LISP loop)
• (return expr): same as (throw nil expr)
• (do init exit . body)
  

  

  The execution of a do loop takes place as follows:

  1. initialize var with expr via let
  2. if , return (progn exprexpr)
  3. if , execute body and re-initialize var with expr via psetq; go to 2.