R: Convert a YAML string into R objects

yaml.load {yaml} | R Documentation

## Convert a YAML string into R objects

### Description

Parse a YAML string and return R objects.

### Usage

      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, ...)

### 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)
`input` | a filename or connection; if `input` is a filename, that file must be encoded in UTF-8
`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

### 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 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.

### 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.

### Author(s)

Jeremy Stephens <jeremy.f.stephens@vumc.org>

### References

YAML: http://yaml.org

libyaml: http://pyyaml.org/wiki/LibYAML

YAML merge specification: http://yaml.org/type/merge.html

### See Also

`as.yaml`

### Examples

      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"))

[Package yaml version 2.2.1 Index]