{"id":344,"date":"2011-08-29T00:30:58","date_gmt":"2011-08-29T04:30:58","guid":{"rendered":"http:\/\/www.peteonsoftware.com\/?p=344"},"modified":"2024-03-02T16:13:01","modified_gmt":"2024-03-02T21:13:01","slug":"my-introduction-to-scheme-part-2","status":"publish","type":"post","link":"https:\/\/www.peteonsoftware.com\/index.php\/2011\/08\/29\/my-introduction-to-scheme-part-2\/","title":{"rendered":"My Introduction to Scheme – part 2"},"content":{"rendered":"

In my last post<\/a>, I started to look at the Scheme language. We finished up with a simple if statement. Its syntax is like this: (if (condition) result1 result2). <\/p>\n

In our last example, “result1” and “result2” were simple return types. If you wanted, the “result1” or “result2” could also be functions that are evaluated to get a return value. Let’s look at that. <\/p>\n

In this case, if 4 is greater than 0, we are going to multiply 4 times 4, if 4 isn’t greater than 0, we are just going to add 4 plus 4.<\/p>\n

(if (> 4 0) (* 4 4) (+ 4 4))<\/pre>\n
\"Complex<\/p>\n
You can see that because 4 is greater than 0, it executed the first block which evaluated 4 * 4 which is 16.  Like last time, here is the equivalent C# version to hopefully add clarity.<\/p>\n
\r\nif (4 > 0)\r\n{\r\n     return 4 * 4;\r\n}\r\nelse\r\n{\r\n     return 4 + 4;\r\n}\r\n<\/pre>\n
One of the most powerful things you can do in a functional language is – of course – to define your own functions.  Here is a simple function that defines a function named pete that squares a number passed to it.<\/p>\n
(define pete (lambda (x) (* x x)))<\/pre>\n
So here, we are calling the define language construct (which is how you declare variables, functions, etc in Scheme) and defining “pete” which is a lambda that takes one parameter – x – and returns the output of the * function being passed x and x.  You see that if we call the pete function and pass in 7, it returns 49 as you would expect.<\/p>\n
\"Simple<\/p>\n
Functions are first class citizens in Scheme, so you can pass them around as arguments.  Let’s see this example where I’m going to let us “do-stuff-to-four-and-nine”.  My goal here is that we are going to create a function that takes a function as a parameter.  It then calls that function, passing it 4 and 9.<\/p>\n
So, first the function. (I used a hard return in the REPL window so that the screenshot below would definitely fit.  It does not cause a problem in the language to have it there).<\/p>\n
(define do-stuff-to-four-and-nine (lambda (our-func) (our-func 4 9)))<\/pre>\n
\"Defining<\/p>\n
All I’ve done is define “do-stuff-to-four-and-nine” as a function that takes a parameter (our-func) that I then use in the body of the lambda, calling it and passing 4 and 9 as arguments.  Next, let’s define 4 functions that we can pass to our “do stuff” function.<\/p>\n
\"Four<\/p>\n
You should be able to simply read these by now.  All four take two parameters, and are plainly named for the simple operations they perform on them.<\/p>\n
Now, all that is left is to call the “do-stuff” function and pass it each of these functions in turn.<\/p>\n
\"Our<\/p>\n
Interestingly, the function must take the right number of arguments.  If not, the language throws up on you.  Notice here that I define a function that only takes one argument.  Then when I try to pass it in to our “four and nine” function, I get an error.<\/p>\n
\"This<\/p>\n
I hope that you’ve learned a little bit more about how Scheme works and how powerful it can be.  My next post is going to take a look at recursion in Scheme and putting a lot of these lessons together.<\/p>\n","protected":false},"excerpt":{"rendered":"
In my last post, I started to look at the Scheme language. We finished up with a simple if statement. Its syntax is like this: (if (condition) result1 result2). In our last example, “result1” and …<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[62],"tags":[118],"class_list":["post-344","post","type-post","status-publish","format-standard","hentry","category-scheme","tag-scheme"],"_links":{"self":[{"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/posts\/344","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/comments?post=344"}],"version-history":[{"count":0,"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/posts\/344\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/media?parent=344"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/categories?post=344"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.peteonsoftware.com\/index.php\/wp-json\/wp\/v2\/tags?post=344"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}