[plt-scheme] How to get a function from the symbol representingit?

From: Jos Koot (jos.koot at telefonica.net)
Date: Wed Apr 9 17:35:51 EDT 2008

Carl Eastlund's remarks are very to the point.

I only want to add that in PLT Scheme you have namespace-variable-value and 
namespace-anchor->namespace (see recent example in a post by Eli Barzilay) 
and may be even more stuff.. They wont give you access to a local 
environment though, i.e. to bindings made by lambda, let, etc. You could, of 
course, redefine all binding forms such as to record their bindings at run 
time, but then you are sort of writing your own interpreter, which can be 
fun, of course, but performance will be terrible.

There is one case in which I use eval: test units that should recover from 
syntax errors, particularly for tests on appropriate syntax error detection 
and report by home made macros) I see no other way. Is there?

----- Original Message ----- 
From: "Carl Eastlund" <cce at ccs.neu.edu>
To: "Grant Rettke" <grettke at acm.org>
Cc: "PLT-Scheme" <plt-scheme at list.cs.brown.edu>
Sent: Wednesday, April 09, 2008 10:30 PM
Subject: Re: [plt-scheme] How to get a function from the symbol 

> On Wed, Apr 9, 2008 at 4:09 PM, Grant Rettke <grettke at acm.org> wrote:
>> I was reading the comp.lang.lisp post [Two Questions about Functions]
>>  where the poster asks how to evaluate a function, given only a string
>>  representing the function name.
>>  Is this something that happens very often in Scheme programs?
>>  1. Take the string of the function name "max"
>>  2. Get the function max for that function name
>>  3. Apply that function to some arguments
>>  I'm under the impression that the only way you can take a string and
>>  get the function represented by the symbol that the string represents
>>  is to use eval, quasiquote, and string->symbol
>>  (eval `(,(string->symbol "max") 1 2 3 4) (scheme-report-environment 5))
>>  Is that true?
> You are correct that eval is the only way to evaluate a string (or any
> value) as Scheme code.  This is not common in Scheme code, but it is a
> common error by beginners to functional languages.  Novices are not
> used to higher order programming and passing around functions as
> values, but they are used to passing strings around.  So, around goes
> the string, and at the application site it has to magically turn into
> a function.  Eval to the rescue!
> There are other solutions, pretty much any of which are preferable.
> One, pass the function around itself.  Lambda is your friend.  Two, if
> you do want to be able to report names to the user (say if the
> matching function is not found), pass a string or symbol around and
> use a hash table to relate names to the actual function you want.
> Either way, the mapping between values and functions is represented in
> the original program, not in other code to be compiled and evaluated
> later.
> Eval is a major pitfall; it's a heavy-weight solution with a lot of
> subtleties and drawbacks that aren't apparent at first glance.  Avoid
> it at all costs unless your actual task is to take Scheme code and
> evaluate it.  Then, of course, eval is 95-100% of your program.
> -- 
> Carl Eastlund
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