Parse a YAML string and return R objects.
Arguments
- 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).
- 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
Precedence behavior during map merges (see Details).
- merge.warning
Whether or not to warn about ignored key/value pairs during map merges.
- input
A filename or connection; if
inputis a filename, that file must be encoded in UTF-8.- readLines.warn
Logical (default: TRUE). Suppress warnings from readLines used inside read_yaml.
- ...
Arguments to pass to yaml.load.
Value
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.
Details
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 explicitly 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 disabled by default
for security reasons. If a ‘!expr’ tag exists and this is set to
FALSE a warning will occur. Alternately, you can set the option named
‘yaml.eval.expr’ via the options function to turn on
evaluation.
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.
References
YAML: http://yaml.org
libyaml: https://pyyaml.org/wiki/LibYAML
YAML merge specification: http://yaml.org/type/merge.html
Author
Jeremy Stephens jeremy.f.stephens@vumc.org
Examples
yaml.load("- hey\n- hi\n- hello")
#> [1] "hey" "hi" "hello"
yaml.load("foo: 123\nbar: 456")
#> $foo
#> [1] 123
#>
#> $bar
#> [1] 456
#>
yaml.load("- foo\n- bar\n- 3.14")
#> [[1]]
#> [1] "foo"
#>
#> [[2]]
#> [1] "bar"
#>
#> [[3]]
#> [1] 3.14
#>
yaml.load("foo: bar\n123: 456", as.named.list = FALSE)
#> [[1]]
#> [1] "bar"
#>
#> [[2]]
#> [1] 456
#>
#> attr(,"keys")
#> attr(,"keys")[[1]]
#> [1] "foo"
#>
#> attr(,"keys")[[2]]
#> [1] 123
#>
if (FALSE) { # \dontrun{
# reading from a file (uses readLines internally)
filename <- tempfile()
cat("foo: 123", file=filename, sep="\n")
yaml.load_file(filename)
} # }
# custom scalar handler
my.float.handler <- function(x) { as.numeric(x) + 123 }
yaml.load("123.456", handlers=list("float#fix"=my.float.handler))
#> [1] 246.456
# custom sequence handler
yaml.load("- 1\n- 2\n- 3", handlers=list(seq=function(x) { as.integer(x) + 3 }))
#> [1] 4 5 6
# custom map handler
yaml.load("foo: 123", handlers=list(map=function(x) { x$foo <- x$foo + 123; x }))
#> $foo
#> [1] 246
#>
# handling custom types
yaml.load("!sqrt 555", handlers=list(sqrt=function(x) { sqrt(as.integer(x)) }))
#> [1] 23.55844
yaml.load("!foo\n- 1\n- 2", handlers=list(foo=function(x) { as.integer(x) + 1 }))
#> [1] 2 3
yaml.load("!bar\none: 1\ntwo: 2", handlers=list(bar=function(x) { x$one <- "one"; x }))
#> $one
#> [1] "one"
#>
#> $two
#> [1] 2
#>
# loading R expressions
# NOTE: this will not be done by default in the near future
doc <- yaml.load("inc: !expr function(x) x + 1", eval.expr=TRUE)
doc$inc(1)
#> [1] 2
# adding a label to error messages
try(yaml.load("*", error.label = "foo"))
#> Error in yaml.load("*", error.label = "foo") :
#> (foo) Scanner error: while scanning an alias at line 1, column 1 did not find expected alphabetic or numeric character at line 1, column 2
#>