quasiquote: Partial quoting in expressions

In wwbrannon/schemeR: Ports of Lisp/Scheme Features to R

Description

A reimplementation of the backquote macro from Lisp/Scheme, with more functionality than the base bquote function. quasiquote quotes its argument, except for two kinds of user-requested evaluation and interpolation. Adapted from base R's bquote.

Usage

quasiquote(expr, where = parent.frame())

Arguments

expr	The expression to be partially quoted.
where	The environment in which any evaluation should occur.

Details

Unlike the bquote() found in base R, this version handles both of the types of unquoting Lisp provides:

• terms wrapped in .c() are evaluated in the environment given by where. This feature replicates Lisp's unquote, or the comma notation, and behaves identically to bquote's .().

• terms wrapped in .s() are evaluated in the environment given by where, are assumed to evaluate to a list, pairlist or vector, and its elements are spliced into the expression where the .s() occurred. This feature replicates Lisp's unquote-splicing, or the comma-at notation.

Value

The quoted expr, with partial evaluation and substitution done.

Warning

Splicing unquote via .s() works on a purely lexical basis, and unlike in Lisp, there is no guarantee the resulting object will make any sense.

Examples

f <- list(1,2,3)
eval(quasiquote(list(.c(f), 4, 5, 6)))

f <- list(5,6,7)
eval(quasiquote(list(1, 2, 3, 4, .s(f), 8, 9, 10)))

wwbrannon/schemeR documentation built on May 4, 2019, 12:03 p.m.