grapes-then-grapes: Applying parsers in sequence

In parcr: Construct Parsers for Structured Text Files

Applying parsers in sequence

Description

(p1 %then% p2) recognizes anything that p1 and p2 would if applied in succession.

Usage

p1 %then% p2

Arguments

p1, p2

two parsers.

Value

A parser.

Pseudocode

(p1 %then% p2)(x):
  if p1(x)==[] or x==null then fail()(x)
  else
    if p2(x[-1])==[] then fail()(x)
    else succeed([p1(x)$L, p2(x[-1])$L])(x[-2])

where null is the empty vector, x[-1] and x[-2] are the vector x without the first element and without the first two elements, respectively.

See Also

The discarding versions %xthen% and %thenx%

Examples

starts_with_a <- function(x) grepl("^a",x[1])
starts_with_b <- function(x) grepl("^b",x[1])
(satisfy(starts_with_a) %then% satisfy(starts_with_b)) (c("ab", "bc", "de")) # success
(satisfy(starts_with_a) %then% satisfy(starts_with_b)) (c("bb", "bc", "de")) # failure
(satisfy(starts_with_a) %then% satisfy(starts_with_b)) (c("ab", "ac", "de")) # failure

parcr documentation built on June 22, 2024, 10:31 a.m.