succeed: The most basic parsers

In parcr: Construct Parsers for Structured Text Files

The most basic parsers

Description

These are the most basic constructors of a parser, but they are important cogs of the parser machinery. The succeed parser always succeeds, without consuming any input, whereas the fail parser always fails.

Usage

succeed(left)

fail(lnr = LNR())

Arguments

left	any R-object constructed from a parsed vector.
lnr	integer. The line number (element number) at which the fail occurs

Details

The succeed parser constructs a list object with a 'left' or L-element that contains the parser result of the consumed part of the input vector and the 'right' or R-element that contains the unconsumed part of the vector. Since the outcome of succeed does not depend on its input, its result value must be pre-determined, so it is included as a parameter.

While succeed never fails, fail always does, regardless of the input vector. It returns the empty list list() to signal this fact.

Value

A list. succeed() returns a list with two elements named L and R. fail() returns a marker object which is basically an empty list with a line number n as attribute. It is printed as the icon ⁠[]⁠, see print.marker(). Note that n will only correctly represent the line number of failure when a parser is wrapped in the reporter() function.

Pseudocode

succeed(y)(x): [L=[y],R=[x]]
fail()(x):     []

where ⁠[L=[y],R=[x]]⁠ is a named list with lists ⁠[y]⁠ and ⁠[x]⁠ as elements and ⁠[]⁠ is an empty list.

Note

It is very unlikely that you will ever have to use these functions when constructing parsers.

Examples

succeed("A")("abc")
succeed(data.frame(title="Keisri hull", author="Jaan Kross"))(c("Unconsumed","text"))

fail()("abc")

parcr documentation built on Aug. 8, 2025, 6:20 p.m.