Reasoning about functions with restricted argument types.

restricted-functions

#+TITLE: Restricted Functions

Many functions in Common Lisp are defined for a wide range of argument
types.  Yet their behavior (in terms of performance, memory consumption and
return value types) varies a lot depending on how many arguments of which
type are supplied.  This library provides /restricted functions/, i.e.,
functions that have been narrowed to a particular set of argument types.
Restricted functions can be created with the function =restrict=

#+BEGIN_SRC lisp
(restrict nil #'+ 'double-float '(complex short-float))
#+END_SRC

Each restricted function is an instance of =funcallable-standard-class=,
and can be queried for a wide range of metadata, such as the number and
type of the values it returns, its name, or its arity.  The most important
feature is the automatic derivation of argument types.  This library can
infer the type of a wide range of functions from the Common Lisp standard.
Type inference can be performed with the function =infer-type=:

#+BEGIN_SRC lisp
(infer-type nil #'* '(integer 2 14) '(integer 0 12)) ; => (integer 0 168)
#+END_SRC

A future goal is to make it possible to query the cost of a restricted
function in terms of CPU cycles, loads, stores and heap allocation on a
given platform and Lisp implementation.

A particularly useful feature is that each restricted function has an
automatically generated name and an optimizing compiler macro for that
name.  This is convenient for writing code generators, where one can just
write

#+BEGIN_SRC lisp
(let ((rf (restrict nil #'+ 'double-float 'double-float)))
  (compile
   nil
   `(lambda (x y)
      (,(name rf) x y))))
#+END_SRC

to get the equivalent of

#+BEGIN_SRC lisp
(compile
 nil
 `(lambda (x y)
    (the double-float (+ (the double-float x) (the double-float y)))))
#+END_SRC

If you have any suggestions for this library, feel free to contact me!


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