[plt-scheme] How to apply a multi-argument function to each element of a list

On Mar 2, 2010, at 8:45 PM, Barry Brown wrote:

> On Mar 2, 2010, at 5:19 PM, Matthias Felleisen wrote:
> 
>> 1. Use #lang scheme instead of damn-big-language. 
>> 
> I see this over and over on the mailing list. I must not have been paying attention when the usage became common. What advantage does "#lang scheme" have over selecting the language from the Language menu?

Look for 'Module' in the language dialogue. A module starts with 

  #lang 

followed by the language you wish to use. In your case, use scheme. (One day, try lazy.) 


>> 2. I also recommend this pattern: 
>> 
>> (define (encrypt n m)
>> (local ((define (encrypt-message msg) ... msg ... n ... m ...))
>>   encrypt-message))
>> 
>> (map (encrypt 17 30) (make-list 100000))
> 
> The tradeoff here is explaining the "local" syntax instead of the "curry" concept. I'll have to think about which would be more appropriate, since the students have seen neither.

A local definite is like a regular define except that the name is visible only from opening ( to closing ). That's it. In many cases, you can go back and forth between local and global definitions with copy-and-paste. 

The novel concept is that a function (name) doesn't have to appear to the right of ( and thus becomes a value in its own right. I switched from lambda to this way of introducing 1st-class functions when I realized that kids like naming things. 

-- Matthias