Convert string to code in Scheme

问题:

How do I convert a string into the corresponding code in PLT Scheme (which does not contain the string->input-port method)? For example, I want to convert this string:

"(1 (0) 1 (0) 0)"

into this list:

'(1 (0) 1 (0) 0)

Is it possible to do this without opening a file?

回答1:

Scheme has procedure read for reading s-expressions from input port and you can convert a string to input stream with string->input-port. So, you can read a Scheme object from a string with

(read (string->input-port "(1 (0) 1 (0) 0)"))

I don't have Scheme installed, so I only read it from reference and didn't actually test it.

回答2:

From PLT Scheme manual:

(open-input-string string [name-v]) creates an input port that reads bytes from the UTF-8 encoding (see section 1.2.3) of string. The optional name-v argument is used as the name for the returned port; the default is 'string.

回答3:

From this similar question on comp.lang.scheme you can save the string to a file then read from it.

That might go something like this example code:

(let ((my-port (open-output-file "Foo")))
  (display "(1 (0) 1 (0) 0)" my-port)
  (close-output-port my-port))

(let* ((my-port (open-input-file "Foo"))
       (answer (read my-port)))
  (close-input-port my-port)
  answer)

回答4:

Many schemes have with-input-from-string str thunk that executes thunk in a context where str is the standard input port. For example in gambit scheme:

(with-input-from-string "(foo bar)" (lambda () (read)))

evaluates to:

(foo bar)

The lambda is necessary because a thunk should be a procedure taking no arguments.