A character is "graphic" if it takes up room on the display surface: thus a letter, number, punctuation mark, symbol, combining character (none in ASCII), or any of Unicode's various space separator characters. It is "printable" if it also leaves ink on the display surface, which excludes the last group. (Unicode does not define "printable", but ISO C and C++ do.)
The current SRFI 175 draft gives the procedures:
* (ascii-display? char)
* (ascii-control->display char)
* (ascii-display->control char)
A possibly more standard term for "display character" is "printable
character". An even more widely used term is "graphic character".
Display character is a term I invented to match Scheme's standard
`display` procedure.
IIRC there may be a subtle distinction between "graphic" and "printable"
characters (one of those sets contains one or more whitespace
characters, whereas the other does not?)
Which term should we use for SRFI 175? Is similar terminology planned to
go into R7RS-large Unicode procedure names?
The precise definition of `ascii-display?` is also up for debate.
Currently space is counted as a displayable character since that's the
most widely used interpretation as far as I can tell. It also makes it
simple to do something like: (when (string-every ascii-display? s)
(display s))