8.3 Reading and Writing Scheme Data

As noted throughout Built-In Datatypes, Scheme provides two ways to print an instance of a built-in value:

• write, which prints a value in the same way that is it printed for a REPL result; and

• display, which tends to reduce a value to just its character or byte content – at least for those datatypes that are primarily about characters or bytes, otherwise it falls back to the same output as write.

Here are some examples using each:

> (write 1/2)   1/2   > (write #\x)   #\x   > (write "hello")   "hello"   > (write #"goodbye")   #"goodbye"   > (write '|dollar sign|)   |dollar sign|   > (write '("alphabet" soup))   ("alphabet" soup)   > (write write)   #<procedure:write>

> (display 1/2)   1/2   > (display #\x)   x   > (display "hello")   hello   > (display #"goodbye")   goodbye   > (display '|dollar sign|)   dollar sign   > (display '("alphabet" soup))   (alphabet soup)   > (display write)   #<procedure:write>

The printf function supports simple formatting of data and text. In the format string supplied to printf, ~a displays the next argument, while ~s writes the next argument.

Examples:

(define (deliver who what)     (printf "Value for ~a: ~s" who what))

> (deliver "John" "string")

Value for John: "string"

An advantage of write, as opposed to display, is that many forms of data can be read back in using read.

Examples:

> (define-values (in out) (make-pipe))

> (write "hello" out)

> (read in)

"hello"

> (write '("alphabet" soup) out)

> (read in)

("alphabet" soup)

> (write #hash((a . "apple") (b . "banana")) out)

> (read in)

#hash((a . "apple") (b . "banana"))