How to capture the return value of `string-search-forward` in Scheme?

I want to write a procedure (function) that checks if a string contains another string. I read the documentation of string library from http://sicp.ai.mit.edu/Fall-2004/manuals/scheme-7.5.5/doc/sche开发者_如何学Cme_7.html

According from to them,

Pattern must be a string. Searches string for the rightmost occurrence of the substring pattern. If successful, the index to the right of the last character of the matched substring is returned; otherwise, #f is returned.

This seemed odd to me, cause the return value is either integer or boolean, so what should I compare my return value with?

I tried

(define (case-one str)
  (if (= #f (string-search-forward "me" str))
      #t
      #f))

DrScheme don't like it,

expand: unbound identifier in module in: string-search-forward

Thanks,

---

string-search-forward is not a standardized Scheme procedure; it is an extension peculiar to the MIT-Scheme implementation (that's why your link goes to the "MIT Scheme Reference Manual.") To see only those procedures that are guaranteed, look at the R5RS document.

In Scheme, #f is the only value that means "false," anything else when used in a conditional expression will mean "true." There is therefore no point in "comparing" it to anything. In cases like string-search-forward that returns mixed types, you usually capture the return value in a variable to test it, then use it if it's non-false:

(let ((result (string-search-forward "me" str)))
  (if result
      (munge result) ; Execute when S-S-F is successful (result is the index.)
      (error "hurf") ; Execute when S-S-F fails (result has the value #f.)
))

A more advanced tactic is to use cond with a => clause which is in a sense a shorthand for the above:

(cond ((string-search-forward "me" str) => munge)
      (else (error "hurf")))

Such a form (<test> => <expression>) means that if <test> is a true value, then <expression> is evaluated, which has to be a one-argument procedure; this procedure is called with the value of <test> as an argument.

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Scheme has a very small standard library, which is both a blessing (you can make small scheme implementations to embed in an application or device, you can learn the language quickly) and a curse (it's missing a lot of useful functions). string-search-forward is a non-standard function of MIT Scheme, it's not present in DrScheme.

Many library additions are available in the form of SRFIs. An SRFI is a community-adopted extension to the base language — think of it as an optional part of a Scheme implementation. DrScheme (or at least its successor Racket) implements many SRFIs.

DrScheme has a number of string functions as part of SRFI 13. Amongst the string searching functions, there is string-contains, which is similar except that it takes its arguments in the opposite order.

(require srfi/13)
(define (case-one str)
  (integer? (string-contains str "me")))

You'll notice that the two implementations used a different argument order (indicating that they were developed independently), yet use the same return value. This illustrates that it's quite natural in Scheme to have a function return different types depending on what it's conveying. In particular, it's fairly common to have a function return a useful piece of information if it can do its job, or #f if it can't do its job. That way, the function naturally combines doing its job (here, returning the index of the substring) with checking whether the job is doable (here, testing whether the substring occurs).

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Error message seems a little odd (I don't have drscheme installed unfortunately so can't investigate too much).

Are you sure str is a string?

Additionally = is for integer comparisons only, you can use false? instead.

As for the return value of string-search-forward having mixed types, scheme has the mindset that if any useful value can be returned it should be returned, so this means different return types are common for functions.

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Try using srfi-13's string-index: http://docs.racket-lang.org/srfi-std/srfi-13.html#Searching The documentation you are looking at isn't specifically for PLT. and probably corresponds to some other version of Scheme.