purrr (version 0.2.5)

as_mapper: Convert an object into a mapper function


as_mapper is the powerhouse behind the varied function specifications that most purrr functions allow. It is an S3 generic. The default method forwards its arguments to rlang::as_function().


as_mapper(.f, ...)

# S3 method for character as_mapper(.f, ..., .null, .default = NULL)

# S3 method for numeric as_mapper(.f, ..., .null, .default = NULL)

# S3 method for list as_mapper(.f, ..., .null, .default = NULL)



A function, formula, or atomic vector.

If a function, it is used as is.

If a formula, e.g. ~ .x + 2, it is converted to a function. There are three ways to refer to the arguments:

  • For a single argument function, use .

  • For a two argument function, use .x and .y

  • For more arguments, use ..1, ..2, ..3 etc

This syntax allows you to create very compact anonymous functions.

If character vector, numeric vector, or list, it is converted to an extractor function. Character vectors index by name and numeric vectors index by position; use a list to index by position and name at different levels. Within a list, wrap strings in get-attr() to extract named attributes. If a component is not present, the value of .default will be returned.


Additional arguments passed on to methods.

.default, .null

Optional additional argument for extractor functions (i.e. when .f is character, integer, or list). Returned when value is absent (does not exist) or empty (has length 0). .null is deprecated; please use .default instead.


Run this code
as_mapper(~ . + 1)

as_mapper(c("a", "b", "c"))
# Equivalent to function(x) x[["a"]][["b"]][["c"]]

as_mapper(list(1, "a", 2))
# Equivalent to function(x) x[[1]][["a"]][[2]]

as_mapper(list(1, attr_getter("a")))
# Equivalent to function(x) attr(x[[1]], "a")

as_mapper(c("a", "b", "c"), .null = NA)
# }

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