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Build an error message from parts

Source: R/cnd-message.R
cnd_message.Rd

cnd_message() assembles an error message from three generics:

  • cnd_header()

  • cnd_body()

  • cnd_footer()

Methods for these generics must return a character vector. The elements are combined into a single string with a newline separator. Bullets syntax is supported, either through rlang (see format_error_bullets()), or through cli if the condition has use_cli_format set to TRUE.

The default method for the error header returns the message field of the condition object. The default methods for the body and footer return the the body and footer fields if any, or empty character vectors otherwise.

cnd_message() is automatically called by the conditionMessage() for rlang errors, warnings, and messages. Error classes created with abort() only need to implement header, body or footer methods. This provides a lot of flexibility for hierarchies of error classes, for instance you could inherit the body of an error message from a parent class while overriding the header and footer.

Usage

cnd_message(cnd, ..., inherit = TRUE, prefix = FALSE)

cnd_header(cnd, ...)

cnd_body(cnd, ...)

cnd_footer(cnd, ...)

Arguments

cnd

A condition object.

...

Arguments passed to methods.

inherit

Wether to include parent messages. Parent messages are printed with a "Caused by error:" prefix, even if prefix is FALSE.

prefix

Whether to print the full message, including the condition prefix (Error:, Warning:, Message:, or Condition:). The prefix mentions the call field if present, and the srcref info if present. If cnd has a parent field (i.e. the condition is chained), the parent messages are included in the message with a Caused by prefix.

Overriding header, body, and footer methods

Sometimes the contents of an error message depends on the state of your checking routine. In that case, it can be tricky to lazily generate error messages with cnd_header(), cnd_body(), and cnd_footer(): you have the choice between overspecifying your error class hierarchies with one class per state, or replicating the type-checking control flow within the cnd_body() method. None of these options are ideal.

A better option is to define header, body, or footer fields in your condition object. These can be a static string, a lambda-formula, or a function with the same signature as cnd_header(), cnd_body(), or cnd_footer(). These fields override the message generics and make it easy to generate an error message tailored to the state in which the error was constructed.