Lisp and Haskell (2017)

Re:Lisp and Haskell (2017) by waterhouse on Saturday July 14, 2018 @11:49PM (#17533697)

For what it's worth, on a fairly recent SBCL, here's what the compiler/interpreter prints when you enter the author's function:

  ~> sbcl
  This is SBCL 1.4.7, an implementation of ANSI Common Lisp.
  More information about SBCL is available at <http://www.sbcl.org/>.
  
  SBCL is free software, provided as is, with absolutely no warranty.
  It is mostly in the public domain; some portions are provided under
  BSD-style licenses.  See the CREDITS and COPYING files in the
  distribution for more information.
  * (defun add-text-padding (str &key padding newline)
    "Add padding to text STR. Every line except for the first one, will be
  prefixed with PADDING spaces. If NEWLINE is non-NIL, newline character will
  be prepended to the text making it start on the next line with padding
  applied to every single line."
    (let ((str (if newline
                   (concatenate 'string (string #\Newline) str)
                   str)))
      (with-output-to-string (s)
        (map 'string
             (lambda (x)
               (princ x s)
               (when (char= x #\Newline)
                 (dotimes (i padding)
                   (princ #\Space s))))
             str))))
  ; in: DEFUN ADD-TEXT-PADDING
  ;     (MAP 'STRING
  ;          (LAMBDA (X)
  ;            (PRINC X S)
  ;            (WHEN (CHAR= X #\Newline) (DOTIMES (I PADDING) (PRINC #\  S))))
  ;          STR)
  ; --> TRULY-THE SB-KERNEL:%MAP
  ; ==>
  ;   (MAP-INTO (MAKE-SEQUENCE SB-C::RESULT-TYPE (MIN (LENGTH #:G51))) SB-C::FUN
  ;             #:G51)
  ;
  ; caught WARNING:
  ;   The function (LAMBDA (X) :IN ADD-TEXT-PADDING) called by MAP-INTO returns NULL but CHARACTER is expected
  ;   See also:
  ;     The SBCL Manual, Node "Handling of Types"
  ;
  ; compilation unit finished
  ;   caught 1 WARNING condition
  
  ADD-TEXT-PADDING
  *

Thus, it identifies the exact problem the author describes at the end.

[ Parent | on HN ]

Re:Lisp and Haskell (2017) by fiddlerwoaroof on Sunday July 15, 2018 @12:52AM (#17533856)

Yeah, just because the language doesn’t force you to specify types, doesn’t mean that the compiler can’t infer them for you.

What was most eye-opening for me about CL was just how helpful the compiler of a dynamically typed programming language can be.

[ Parent | on HN ]

Re:Lisp and Haskell (2017) by fjsolwmv on Sunday July 15, 2018 @01:48AM (#17533973)

Checking dynamic types us not the work of a compiler. Checking types at compile time is static type checking. If a type is wholly dynamic (determined at runtime via something like casting to a type specified in user input), the compiler can't check it.

(Also, type inference is orthogonal to static vs dynamic typing.)

[ Parent | on HN ]

Re:Lisp and Haskell (2017) by phoe-krk on Sunday July 15, 2018 @02:22AM (#17534053)

> Checking dynamic types us not the work of a compiler. Checking types at compile time is static type checking.

By your logic, Common Lisp is both dynamically and statically typed, because compile-time type checks are done by the compiler and warnings/errors are signaled whenever proper.

More, if you declare (OPTIMIZE SPEED), the compiler is going to print a list of all places where it was unable to infer types on its own and where it expects help from the programmer in form of type declarations for individual variables.

[ Parent | on HN ]

Re:Lisp and Haskell (2017) by kazinator on Sunday July 15, 2018 @01:00AM (#17533871)

Also: why would padding be an optional keyword parameter (defaulting to nil!) when it's required to be a non-negative integer for the function to work correctly?

Since it is required, it wants to be a required parameter.

[ Parent | on HN ]