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Declare how parts are combined

Source: R/resource.R
combiner.Rd

A combiner turns the verified part()s of a resource, in declaration order, into the single artefact the resource names. Concatenation is the default and needs no declaration; a combiner is what a delta format calls for, since applying a patch is knowledge about a file format and getaca has none.

Usage

combiner(id, fn)

Arguments

id

Short stable identifier for this transformation, for example "bsdiff".

fn

A function (parts, output), where parts is a character vector of verified local paths in declaration order and output is the file to write. The return value is ignored.

Value

An object of class getaca_combiner.

Details

The result is held to the resource's own SHA-256 like any other bytes, so a combiner cannot produce something other than what the declaration promises. That is also why the manifest records a combiner by id alone: the checksum says the result is right, and the identifier only says what to run.

See also

part()

Examples

combiner("bsdiff", function(parts, output) {
  # apply parts[-1] to parts[1], writing the result to output
  file.copy(parts[1], output)
})