29 - Functional

29 - Functional#

Characters & Character Sets#

  • #\a, #\A, #\(

  • #\space, #\newline

  • char-set:alphanumeric

  • char-set:whitespace

  • (char-set-member? alphanumeric R) - Checks is R is in char-set:alphanumeric

Strings#

  • "abc", "This is a string"

  • (string-length "The length")

  • (string=? string1 string2)

  • (string-ci=? string1 string2) - case insensitive

  • (string-capitalize string) - capitalize the first letter

  • (string-ref string k)

  • (substring string start end)

  • (string-append string ...)

Ports#

  • A port serves as a source or sink for data

  • A port must be open before it can be read from or written to

  • (open-input-file filename)

  • (close-input-port port)

  • (open-output-file filename)

  • (close-output-port port)

Read a Character#

  • (read-char [input-port]) - port is optional

  • (peek-char [input-port])

  • (eof-object? object) - check if it reaches eof

Read and run the example printfile4.scm with

scheme --quiet < printfile4.scm

Read a Line#

  • (read-line [input-port])

  • (eof-object? object)

Read and run the example printfile3.scm with

scheme --quiet < printfile3.scm

Read an Object#

  • (read [input-port]) - Read external representation of next Scheme object (number, symbol, list) and return it

    • Note that a symbol, different from a string, will be stored in lowercase

  • (eof-object? object)

Read and run the example printfile2.scm with

scheme --quiet < printfile2.scm

Read a String#

  • (read-string char-set [input-port])

  • (eof-object? object)

  • read-string works only on Scheme version 9.2 (which we use), not the most recent version 11.2, which uses read-delimited-string

Read and run the example printfile.scm with

scheme --quiet < printfile.scm

Output Ports#

  • (write-char output-port char)

  • (write-string output-port string)

  • (write-substring output-port string start end)

  • (write-line output-port string)

  • (write output-port object)

String Ports#

  • (open-input-string string [start [end]])

  • First, open-input-string, then write, then finally get-output-string port to get the combined string

  • Similar to file input/output

Read and run the example parsestring.scm with

scheme --quiet < parsestring.scm

Recursion#

Recursion is used instead of iteration in functional languages.

Here is recursion to skip all the white spaces:

(define (skip-whitespaces port)
    (let ((ch (peek-char port)))
        (if (and (not (eof-object? ch)) (char-whitespace? ch))
            (begin (read-char port)
                (skip-whitespaces port)
            )
        )
    )
)

Sequencing#

  • (begin expression expression ...)

  • The expressions are evaluated sequentially from left to right

(begin
    (display "4 plus 1 equals ")
    (display (+ 4 1))
)
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