Let Over Lambda—50 Years of Lisp | Hacker News Reader

Let Over Lambda—50 Years of Lisp

letoverlambda.com

97 pointsananthrk9 comments

very very good book about lisp macros. with pg's onlisp, you don't need any other source.

  with pg's onlisp, you don't need any other source.

Relevant excerpt from the introduction

  There are very few good references or tutorials on macro
  construction. Paul Graham's On Lisp[ON-LISP] is one of the
  exceptions. Every word of On Lisp is required reading for anyone
  interested in macros. On Lisp and Graham's other writings were the
  most important inspirations for the creation of the book you are
  reading now. Thanks to Paul Graham and other lisp writers, the power
  that macros provide programmers is widely discussed, yet is
  unfortunately still widely misunderstood. Despite the wisdom regarding
  macro programming that can be gleaned from a simple perusal of On
  Lisp, few programmers make the connection between the macro and their
  real-life programming problems. While On Lisp will show you the
  different types of macros, this book will show you how to use them.

Having read about half of On Lisp, I think that's a fair claim. I am very glad to be reading it, it is a wonderful book, and thanks to it I know what a macro is and what it can do, but haven't really come away with a good feel for when to use them myself in my own code. (It may be that the logic of my code is too simple to require them.) Whether Lambda Over Let lives up to its claim of showing how to use them, I don't know. Definitely going to take a look.

In the "Macro Basics" section, he seems to completely glaze over the _other_ problem with unhygenic macros.[1]

Consider:

  (defmacro square (x)
    `(* ,x ,x))

  (let ((* my-vector-dot-product))
    ...
    (square (* bar baz)))

He seems a bit brash in his attacks on Scheme and insistence that hygenic macros are bad. He doesn't even mention Clinger's explicit renaming macros which solve the problem without using a DSL (an approach which he derides).

Also, what about side effecting arguments to macros? I'm hesitant to put too much credence in this book when it seems to ignore significant problems with defmacro.

[1] http://letoverlambda.com/index.cl/guest/chap3.html#sec_5

There are two problems with your example. First is that Common Lisp is a lisp-2 so the '*' function won't actually be rebound at all.

Secondarily, and more importantly, every unhygienic lisp I've used has had a broader namespacing system which takes care of this. The compiler warns you whenever you are about to rebind something within the same namespace.

Like, Common Lisp? (Clozure)

    ? (defun plus (&rest x) (apply #'+ x))
    PLUS
    ? (defmacro mplus (&rest x) `(plus ,@x))
    MPLUS
    ? (flet ((plus (&rest x) (apply #'* x))) (mplus 4 9))
    36 ;; Uh oh, looks like my function was rebound
    ? (plus 4 9)
    13

How about functions in the CL package?

    ? (defmacro mplus (x y) `(+ ,x ,y))
    MPLUS
    ? (flet ((+ (x y) (- x y))) (mplus 8 4))
    4

Your second example throws an error for me. (SBCL)

The first doesn't (it appears flet doesn't trigger the error in this situation) but the package system still mitigates the damage in practice. I've written a lot of macros and haven't yet accidentally rebound a special function with flet which a macro relied upon.

> Your second example throws an error for me. (SBCL)

According to the CL standard, you shouldn't be able to redefine or shadow special operators, functions, or macros in the COMMON-LISP package. SBCL does the right thing by the spec, but it's really a weakness of the spec that makes CL less general than it can be.

This restriction and the multiple namespaces of Common Lisp (and given appropriate use of gensym) is why almost all of the time CL macros work without need of hygiene. But there's nothing stopping surprises when you call functions or macros defined in packages other than COMMON-LISP.

I wrote a review a couple of years ago: http://carcaddar.blogspot.com/2009/04/review-let-over-lambda...

Chapters 7 and 8 haven't been posted online; it's worth purchasing the book to read them.

Oops,they shouldn't translate the title into Japanese at News (see about the book), as its sold as "Let over Lambda 1.0" as same as English. Of course contents are translated.

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