[Cs254f11] lets that depend on other things that have been let in the same context

[Lee Spector]

FYI some lisps provide ways to do sequential bindings (like Clojure's "let") and also other ways to do bindings in which the init-expressions can be evaluated in parallel (and so they can't refer to each other). Confusingly for the polylisplingual Clojure's "let" is roughly Common Lisp's "let*" while Common Lisp's "let" does the parallel thing (semantically -- not generally using actual parallel execution).

BTW if you use a binding form that doesn't allow for sequential binding then you could always fake it by nesting calls -- I think that in some lisps the sequential version is actually a macro that expands into a nested call to the non-sequential version.

 -Lee

On Dec 2, 2011, at 1:40 PM, Wm. Josiah Erikson wrote:

> Oh. Right. Duh :)
> 
> I actually even knew that at some point, looking back earlier in my code. Thanks. I knew that couldn't be right - you HAVE to be able to define variables that depend on other variables.
> 
>    -Josiah
> 
> 
> On 12/2/11 12:08 PM, Lee Spector wrote:
>> Josiah,
>> 
>> You just have too many brackets in there. In a "let" you can define multiple bindings, each with its own symbol and init-expression, but you should just list them all in the same single set of [] brackets. So you probably want this:
>> 
>> (defn prediction-test
>>   [psize gens]
>>   (let [best-fit (evolve_best_fit psize gens)
>>         reference-error (error best-fit)
>>         test-error (test_error best-fit)
>>         error-difference (Math/abs (- reference-error test-error))]
>>     (println "The shortest best fit individual over" gens "generations with a population size of" psize)
>>     (println "was" best-fit)
>>     (println "It had an error of" reference-error "against the reference songs, and an error of")
>>     (println test-error "against the test songs, giving a difference of" error-difference)))
>> 
>>  -Lee
>> 
>> 
>> On Dec 2, 2011, at 11:35 AM, Wm. Josiah Erikson wrote:
>> 
>>> I find it annoying, just for readability of my program and not having to do things over and over again, that I can't do this:
>>> 
>>> (defn prediction-test
>>>  [psize gens]
>>>  (let [best-fit (evolve_best_fit psize gens)]
>>>    [reference-error (error best-fit)]
>>>    [test-error (test_error best-fit)]
>>>    [error-difference (Math/abs (- reference-error test-error))]
>>>    (println "The shortest best fit individual over" gens "generations with a population size of" psize)
>>>    (println "was" best-fit)
>>>    (println "It had an error of" reference-error "against the reference songs, and an error of")
>>>    (println test-error "against the test songs, giving a difference of" error-difference)))
>>> 
>>> It tells me that it can't resolve the symbol reference-error in this context. I assume that this is because it defines those out of order or something. But why didn't it complain about not knowing what best-fit was first?
>>> 
>>> Am I doing this wrong, or can you just not do this?
>>> 
>>> -- 
>>> Wm. Josiah Erikson
>>> Network Engineer
>>> Hampshire College
>>> Amherst, MA 01002
>>> (413) 559-6091
>>> 
>>> _______________________________________________
>>> Cs254f11 mailing list
>>> Cs254f11 at lists.hampshire.edu
>>> https://lists.hampshire.edu/mailman/listinfo/cs254f11
>> --
>> Lee Spector, Professor of Computer Science
>> Cognitive Science, Hampshire College
>> 893 West Street, Amherst, MA 01002-3359
>> lspector at hampshire.edu, http://hampshire.edu/lspector/
>> Phone: 413-559-5352, Fax: 413-559-5438
>> 
> 
> -- 
> Wm. Josiah Erikson
> Network Engineer
> Hampshire College
> Amherst, MA 01002
> (413) 559-6091
> 

--
Lee Spector, Professor of Computer Science
Cognitive Science, Hampshire College
893 West Street, Amherst, MA 01002-3359
lspector at hampshire.edu, http://hampshire.edu/lspector/
Phone: 413-559-5352, Fax: 413-559-5438