Does Common Lisp need a better type system?

Discussion of Common Lisp

Re: Does Common Lisp need a better type system?

Postby cperkins » Wed Jul 23, 2008 5:59 pm

I've always wished that CL had at least an option for compile time type checking. Maybe one of the commercial implementations have something.

Chris
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Re: Does Common Lisp need a better type system?

Postby magiuspendragon » Thu Jul 24, 2008 4:00 am

secondly, give me an example of "a new source of abstractions." Again, keep it simple. I'm a below average engineer, not a theorist, remember.


Having moved to CL from ML in many ways, one of the things I find myself wanting to do every once in a while is using types as a method of abstraction, in particular what ML terms algebraic data types (or at least that's how I was taught it).

So a good example of this functionality already in lisp is the list, where you have either a cons cell or nil. This is in many ways a conceptual abstraction in order to better reason about lists. Now one of the things you can do in ML is declare new constructors, so that you can define a tree for example, in the same way. It can be a Node, which has 3 fields in it (a left branch, a value, and a right branch), or a Leaf which has just a value. Now there are many other ways to represent this (such as just having a leaf node be one in which both branches are nil, a la the common c++ definition, or a list of lists a la what i assume is common in lisp systems) but once I saw this other method, I always found it more intuitive.

Now that being said, it is possible to some extent to mimic the dynamic behavior of such constructs by writing the predicates by hand and/or using the deftype form (i believe there are asdf-installable packages which also bundle the work into a couple macros). However, there's a few reasons why I always feel put off by this approach. One main one is that I really don't understand deftype... I keep reading the hyperspec definitions and coming away still not knowing how to correctly use the form. The other reason is that type decls, which can be annoying to include in the first place, are also allowed to be ignored by the complier in implementation dependent ways, which makes the whole behavior questionable when migrating systems.

That being said, there's arguments both for and against static type systems, a la ML / Haskell vs a dynamic type system like lisp. The particular win i see for lisp is exploratory programming is almost prohibitive in a static type system for any problem which would incorporate meaningful type abstractions. However, when not performing exploratory programming (which admittedly is my preference) compile time static type inference provides alot of useful information for debugging purposes, without having to engage in the tedium of using type declarations everywhere, which also many times I feel like I don't understand.

So does lisp need a better type system? In my opinion, there are type constructs that can be useful for thinking about problems, and type inference can help in debugging. But much of that is a personal choice, some people view one or both of those as a hurdle to effective software development. A different type system is one option, though in many cases I'd settle for better documentation of the one already existing. I just never did find it clear.
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Re: Does Common Lisp need a better type system?

Postby metageek » Fri Jul 25, 2008 8:57 am

findinglisp wrote:Whenever I read anything about type systems on Lambda the Ultimate or other more theoretically-oriented sites, my eyes just glaze over and and I start drooling.


Well, yeah. That's because LtU people are off in the future, working on type systems that have powerful capabilities, but haven't yet been reduced a form that one can use and understand without learning category theory first. If you want a feel for what a more advanced type system can do for you, go play with some form of ML. It's not as far along as dependent types and the other drooligens you see on LtU, but it's much nicer than the sorts of things you see in more mainstream languages.
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Re: Does Common Lisp need a better type system?

Postby findinglisp » Fri Jul 25, 2008 9:04 am

metageek wrote:
findinglisp wrote:Whenever I read anything about type systems on Lambda the Ultimate or other more theoretically-oriented sites, my eyes just glaze over and and I start drooling.


Well, yeah. That's because LtU people are off in the future, working on type systems that have powerful capabilities, but haven't yet been reduced a form that one can use and understand without learning category theory first. If you want a feel for what a more advanced type system can do for you, go play with some form of ML. It's not as far along as dependent types and the other drooligens you see on LtU, but it's much nicer than the sorts of things you see in more mainstream languages.


Good suggestion. Thanks, I'll check out ML. That's one language I have never looked at in the slightest.
Cheers, Dave
Slowly but surely the world is finding Lisp. http://www.findinglisp.com/blog/
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Re: Does Common Lisp need a better type system?

Postby nikodemus » Sat Jul 26, 2008 1:29 am

cperkins wrote:I've always wished that CL had at least an option for compile time type checking. Maybe one of the commercial implementations have something.

Hm, what's this then?

Code: Select all
CL-USER> (defun foo (x) (declare (string x)) (setf x (list x)))
; in: LAMBDA NIL
;     (SETF X (LIST X))
; --> SETQ
; ==>
;   (THE #<SB-KERNEL:UNION-TYPE STRING> (LIST X))
;
; caught WARNING:
;   Asserted type STRING conflicts with derived type (VALUES CONS &OPTIONAL).
;   See also:
;     The SBCL Manual, Node "Handling of Types"
;
; compilation unit finished
;   caught 1 WARNING condition
FOO
CL-USER> (defun bar (x) (car (- x)))
; in: LAMBDA NIL
;     (CAR (- X))
;
; note: deleting unreachable code
;
; caught WARNING:
;   Asserted type LIST conflicts with derived type (VALUES NUMBER &OPTIONAL).
;   See also:
;     The SBCL Manual, Node "Handling of Types"
;
; compilation unit finished
;   caught 1 WARNING condition
;   printed 1 note
BAR


Sure, this is not what the static typing crowd calls type checking, but it still looks an awful lot like it to me!

Re. issues with the type system or related parts, in no specific order:

  • Can't declare exact type of array (fill-pointer, adjustability, displacement).
  • Can't declare functions are pure/foldable/flushable.
  • DEFTYPE cannot specify a recursive type, and no SIMPLE-LIST type (where CAR would always be of a specific type).
  • Arrays of element type NIL are strings.
  • Incompabtible re-declamation of a global type of ftype business as usual. No need to forbid it, but requiring consistency *across* different compilation units and a full warning on redefinition would be good.
  • No standard way to extend proper compilation units across files. (WITH-COMPILATION-UNIT deals with warnings, and implementations are not allowed to make it imply other things for multi-file scope without using a non-standard keyword argument.)
  • Subclasses are allowed to restrict their slot types.
  • Non-monotonic class linearization.
  • No sealing.
  • No way to define funcallable structures (or structures with other metaclasses.)
  • No standard environment access to declared types.
  • No CONSTANT-VALUE. (Extract the value of form for which CONSTANTP is true.)
  • No standard way to write optimizers for post-source stages of compilation, where type-inference information is available. (Moral equivalents of SBCL's DEFTRANSFORM and DEFINE-OPTIMIZER DERIVE-TYPE.)

Some of these can be deal with in user-space, some with implementation specific extension -- but some things like subclasses restricting their slot types, and non-monotonicity cannot (Well, they can if you reimplement chunks of CLOS, but that's like saying you can do closures in C). ...how serious such omissions are, is another question entirely.

Cheers,

-- Nikodemus
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