Convert a YAML string into R objects
Parse a YAML string and return R objects.
yaml.load(string, as.named.list = TRUE, handlers = NULL, error.label = NULL,
eval.expr = getOption("yaml.eval.expr", TRUE),
merge.precedence = c("order", "override"), merge.warning = FALSE)
yaml.load_file(input, error.label, ...)string |
the YAML string to be parsed |
as.named.list |
whether or not to return a named list for maps (TRUE by default) |
handlers |
named list of custom handler functions for YAML types (see Details) |
input |
a filename or connection; if |
error.label |
a label to prepend to error messages (see Details) |
eval.expr |
whether or not to evaluate expressions found in the YAML document (see Details) |
merge.precedence |
behavior of precedence during map merges (see Details) |
merge.warning |
whether or not to warn about ignored key/value pairs during map merges |
... |
arguments to pass to yaml.load |
Use yaml.load to load a YAML string. For files and connections, use
yaml.load_file, which calls yaml.load with the contents of the specified
file or connection.
Sequences of uniform data (e.g. a sequence of integers) are converted into vectors. If
the sequence is not uniform, it's returned as a list. Maps are converted into named lists
by default, and all the keys in the map are converted to strings. If you don't want the
keys to be coerced into strings, set as.named.list to FALSE. When it's FALSE, a
list will be returned with an additional attribute named 'keys', which is a list of the
un-coerced keys in the map (in the same order as the main list).
You can specify custom handler functions via the handlers argument. This argument
must be a named list of functions, where the names are the YAML types (i.e., 'int', 'float',
'seq', etc). The functions you provide will be passed one argument. Custom
handler functions for string types (all types except sequence and map) will receive a
character vector of length 1. Custom sequence functions will be passed a list of objects.
Custom map functions will be passed the object that the internal map handler creates, which
is either a named list or a list with a 'keys' attribute (depending on as.named.list).
ALL functions you provide must return an object. See the examples for custom handler use.
You can specify a label to be prepended to error messages via the
error.label argument. When using yaml.load_file, you can
either set the error.label argument explicitly or leave it missing.
If missing, yaml.load_file will make an educated guess for the value
of error.label by either using the specified filename (when
input is a character vector) or using the description of the supplied
connection object (via the summary function). You can explicity set
error.label to NULL if you don't want to use this functionality.
There is a built-in handler that will evaluate expressions that are tagged
with the ‘!expr’ tag. Currently this handler is enabled by default,
but the handler is being disabled by default in an upcoming version for
safety and security reasons. If you do not explicity set the
eval.expr argument to TRUE, you will get a warning if an
expression is evaluated. Alternately, you can set the option named
‘yaml.eval.expr’ via the options function to turn off the
warning.
The merge.precedence parameter controls how merge keys are handled.
The YAML merge key specification is not specific about how key/value
conflicts are resolved during map merges. As a result, various YAML library
implementations vary in merge key behavior (notably Python and Ruby). This
package's default behavior (when merge.precedence is ‘order’)
is to give precedence to key/value pairs that appear first. If you set
merge.precedence to ‘override’, natural map key/value pairs
will override any duplicate keys found in merged maps, regardless of order.
This is the default behavior in Python's YAML library.
This function uses the YAML parser provided by libyaml, which conforms to the YAML 1.1 specification.
If the root YAML object is a map, a named list or list with an attribute of 'keys' is returned. If the root object is a sequence, a list or vector is returned, depending on the contents of the sequence. A vector of length 1 is returned for single objects.
Jeremy Stephens <jeremy.f.stephens@vumc.org>
YAML: http://yaml.org
libyaml: http://pyyaml.org/wiki/LibYAML
YAML merge specification: http://yaml.org/type/merge.html
yaml.load("- hey\n- hi\n- hello")
yaml.load("foo: 123\nbar: 456")
yaml.load("- foo\n- bar\n- 3.14")
yaml.load("foo: bar\n123: 456", as.named.list = FALSE)
## Not run:
# reading from a file (uses readLines internally)
filename <- tempfile()
cat("foo: 123", file=filename, sep="\n")
yaml.load_file(filename)
## End(Not run)
# custom scalar handler
my.float.handler <- function(x) { as.numeric(x) + 123 }
yaml.load("123.456", handlers=list("float#fix"=my.float.handler))
# custom sequence handler
yaml.load("- 1\n- 2\n- 3", handlers=list(seq=function(x) { as.integer(x) + 3 }))
# custom map handler
yaml.load("foo: 123", handlers=list(map=function(x) { x$foo <- x$foo + 123; x }))
# handling custom types
yaml.load("!sqrt 555", handlers=list(sqrt=function(x) { sqrt(as.integer(x)) }))
yaml.load("!foo\n- 1\n- 2", handlers=list(foo=function(x) { as.integer(x) + 1 }))
yaml.load("!bar\none: 1\ntwo: 2", handlers=list(foo=function(x) { x$one <- "one"; x }))
# loading R expressions
# NOTE: this will not be done by default in the near future
doc <- yaml.load("inc: !expr function(x) x + 1")
doc$inc(1)
# adding a label to error messages
try(yaml.load("*", error.label = "foo"))Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.