reading a “.” in scheme R5RS

Question

I need to be able to read user input in scheme for a project. For example, I need to be able to read the string 4 5 * . . I was implementing it using the (read) function but it gives an error when it reads a . . I would use a different symbol but it is specified by the project description. Is there a way to do this?

Answer 1

You cannot use read to input arbitrary text. The read procedure is only meant for inputting "S-expressions", a data format that can be used to represent a superset of Scheme source code expressions.

The reason you cannot read a . via the read procedure is that a period token has a special role in Scheme source: it is used for dotted pair notation. (C1 . C2) is the way that the pair of C1 and C2 is written as an S-expression. Note there is a crucial difference between the single pair (C1 . C2) and the list (C1 C2) (which is made from two pairs); and yet the only difference between the source text is the presence/absence of a single period.

The dotted pair notation is described in section 6.3.2 of the R5RS .

So, as suggested in the comments on your question by Dan D., you should consider using the read-char procedure to consume user input text. It described in section 6.6.2 of the R5RS . It may seem counter-intuitive, since read-char only consumes a single character while read consumes many characters (and builds a potentially large tree of structured data), but the reality is that you can build your own parser on top of read-char , by invoking it repeatedly in a loop, as suggested by Dan D.

In fact, some scheme systems implement read itself by making it a Scheme procedure that invokes read-char . See for example Larceny's reader source code , where read will call get-datum , which calls get-datum-with-source-locations , which calls read-char in a number of places.

Alternatively, you might have other ways of reading input from the user. The read-line procedure is quite common (and its also easy to write on top of read-char ). Or you might look into a Parser-Generator (like the one that generated the source code for Larceny's reader, linked above.