Re: need for macros (was Re: Icon)

[Matthias Felleisen <address@hidden>]

That's a new binding construct. In 

 (with-unique-id var body ...) 

var is bound to something unique in body ... 

-- Matthias

  > Date: Thu, 20 Dec 2001 18:57:45 -0800 (PST)
  > From: Paul Graham <paulgraham@yahoo.com>
  > Content-Type: text/plain; charset=us-ascii
  > 
  > I've never been able to come up with a list of what you
  > can use macros for.  I continue to be surprised by what
  > you can do with them.  For example, at one point in 
  > the Arc code I'm writing I needed to create a block with
  > a unique identifier, so I know which button someone
  > clicked.  The code is going to be evaluated multiple
  > times and the identifier associated with each button has
  > to be the same.  Answer: create the identifier at compile-
  > time:
  > 
  > (mac with-unique-id (var . body)
  >   `(let ,var ',(unique-id)
  >      ,.body))
  > 
  > where unique-id is a function that returns an id-string 
  > guaranteed (since starting Arc) to be unique.  Then you
  > can say in the code that makes the html page
  > 
  > (with-unique-id x
  >   ...
  >   (input type 'submit value "Submit" name x)
  >   ...)
  > 
  > And every button everywhere in your code has a single
  > distinct name.
  > 
  > This use of macros doesn't seem to be on your list.
  > 
  > 
  > --- Matthias Felleisen <matthias@ccs.neu.edu> wrote:
  > > 
  > > la@iki.fi writes: 
  > > 
  > >   Note that there is no need for any syntax transformations, this is
  > > pure
  > >   Haskell. You can get surprisingly far without macros when just
  > > about
  > >   everything is a first-class value...
  > > 
  > > That's not true. There are three reasons for introducing a syntactic
  > > abstraction (macro) rather than a function:
  > > 
  > >  1. new binding forms
  > >  2. implicit quoting or, more generally, a data sub-language
  > >  3. an order of evaluation that is incompatible with evaluation 
  > > 
  > > A lazy language makes macros fro 3 unnecessary. A function with
  > > first-class
  > > functions still needs macros for 1; otherwise you keep writing 
  > >   foo (fn x => ...)
  > > all over the place. 
  > > 
  > > -- Matthias
  > > 
  > >   
  > > 
  > 
  > 
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