[Sbcl-help] Questions about ftype declarations.

[Nathan T. <nb...@nb...>]

I recently went through a library I'm working on and removed all type
declarations on formal parameters from the top of DEFUNs, the reason
being that I assumed such declarations were redundant in light of FTYPE
declarations that also declare the types of the arguments passed to
functions.  And since function type declarations also declare return
types, I figured it made more sense to keep the FTYPE declarations and
remove the TYPE declarations from the DEFUNs.

However, when I ran some tests after removing the declarations, I
noticed that performance had taken a huge hit.  Certain procedures took
almost twice as long as they had taken before I had removed the type
declarations.  (The optimize setting being (optimize (debug 0) (safety
0) speed).)  Can anyone here explain why this might be?  (I figured that
since optimizations aren't defined by the language itself, this would be
the best place to ask.)  Why don't FTYPE declarations on function
arguments have the same effect as TYPE declarations on formal
parameters?  Given that the effect is different, as it seems, when
_should_ one use FTYPE declarations for potential performance
improvements?  Should I be doing both if I want maximal performance?

On a related note, if I declare a function's return type in an FTYPE
declaration and bind a variable using the function, as in:

  (declaim (ftype (function (&rest) <type>)) foo)

  (let ((var (foo a b c)))
    ...)

will the compiler infer that VAR has type <type>, or should I declare
that at the top of the LET's body (assuming I want maximal performance)?

Thank you.

Nathan