Lately I’ve been trying to iron out the details of a generalization of quasiquotation which I call “higher quasiquotation.” The basic idea is that just as a quasiquotation is a region in one parenthesis-delimited region (marked by
quasiquote) and a set of other parenthesis-delimited regions (marked by
unquote), we can go on to talk about regions between quasiquoted regions, regions between those regions, and so on.
If you think of values with holes as being functions, then the notion that this is a “higher-order” quasiquotation is clear: Each quasiquotation determines a value of type
(c SExpr -> SExpr), the next higher degree of quasiquotation determines a value of type
(c (c SExpr -> SExpr) -> (c SExpr -> SExpr)), and so on, where
c is some collection like
c a = Map String a. But these functions aren’t the whole story; the quasiquotations should be able to be pulled apart like other data structures, not just filled in to create s-expressions.
I haven’t managed to write a full macroexpander for higher quasiquotation yet. I’ve written this post to share my status as it is.