How do I evaluate a symbol returned from a function in Scheme?

Question

I'm refamiliarizing myself with Scheme and I've hit a problem that is probably reflecting a fundamental misunderstanding on my part.

Say I do the following in Scheme (using Guile in this case but it's the same in Chicken):

> (define x 5)
> x
5
> (string->symbol "x")
x
> (+ 5 (string->symbol "x"))
<unnamed port>:45:0: In procedure #<procedure 1b84960 at <current input>:45:0 ()>:
<unnamed port>:45:0: In procedure +: Wrong type: x
> (symbol? (string->symbol "x"))
#t
> (+ 5 x) ; here x is dereferenced to its value 5
10
> (+ 5 'x) ; here x is not dereferenced
<unnamed port>:47:0: In procedure #<procedure 1c7ba60 at <current input>:47:0 ()>:
<unnamed port>:47:0: In procedure +: Wrong type: x

I understand that string->symbol is returning a symbol, x, which is effectively quoted. However, I cannot figure out how to use the symbol returned by string->symbol in any later context. How can I have Scheme evaluate that symbol?

To give a background on why I want to do this, it's that I'm writing a C program with embedded Guile. I would like to be able to be able to access symbols defined in Guile by name from C, using for example scm_from_*_symbol or scm_string_to_symbol. The reasons these functions aren't working the way I thought they would is related to my core question above. Perhaps there's a better way to do what I want to do with Guile, but that's a different question. Right now I'm interested in the fundamental question above.

Óscar López · Answer 1 · 2013-06-02T23:23:07.343

3

What you want is to evaluate the symbol (not to "dereference" it). I think this is what you meant:

(define x 5)
(+ 5 (eval 'x (interaction-environment)))
=> 10

Take a look at the documentation for further details.

edited Jun 02 '13 at 23:23

answered Jun 01 '13 at 13:19

Óscar López

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That don't work on Guile 2.0, because its `eval` wants *two* arguments (the thing to evaluate, and the environment....) – Basile Starynkevitch Jun 01 '13 at 14:30
Doen't this example have useless evaluation of number? x is evaluated to 5 before eval is applied. – Sylwester Jun 02 '13 at 19:41

kirelagin · Answer 2 · 2014-01-28T20:02:12.430

2

Symbols are not special in this sense, that is they are not easier to evaluate than ordinary strings.

Symbol is much like a string, it just doesn't have quotes around it. Well, the fundamental difference is, of course, not absence of quotes, but the fact that symbols are interned. This means that strings "x" and "x" are two different strings (although they are equal), while symbols 'x and 'x are actually the same object.

edited Jan 28 '14 at 20:02

answered Jun 01 '13 at 13:10

kirelagin

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Hm, then it is a fundamental misunderstanding. I corrected my question title to say "dereference", which is what I meant. – Jun 01 '13 at 13:12
@BrandonInvergo Again, you really can think that symbols are string just without quotes. `(+ 5 "x")` makes no sense, so does `(+ 5 'x)`. – kirelagin Jun 01 '13 at 13:14
If you want to evaluate contents of a string as Scheme code, you don't need symbols, you need `eval-string` function. – kirelagin Jun 01 '13 at 13:15
Actually yes, it's possible to _evaluate_ a symbol. – Óscar López Jun 01 '13 at 13:28
@ÓscarLópez Right, I've changed wording in my answer to be more precise. – kirelagin Jun 01 '13 at 15:00

score 2 · Accepted Answer · answered Jun 01 '13 at 13:10

2

You should read the chapter fly-evaluation of the Guile documentation.

You want eval and probably interaction-environment

I recommend reading the famous SICP and Queinnec's Lisp In Small Pieces

answered Jun 01 '13 at 13:10

Basile Starynkevitch

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`eval` did exactly what I wanted. Strange, because I tried that before but I guess I did it wrong at first. Thanks! – Jun 01 '13 at 13:16

How do I evaluate a symbol returned from a function in Scheme?

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