result

The result type captures expressions and functions that might cause error or otherwise fail to produce a value. Using result can make code more readable and less error-prone by abstracting away the need for nested if statements and tryCatch blocks. At its most powerful, result can be used to build a composable pipeline of functions that will either end with a success object, or stop gracefully with a failure object as soon as an error is detected.

Example

Safe functions

Make your own functions fault-tolerant by wrapping your results in success or failure for downstream users to safely use:

library(result)

# A function that might fail
risky_fn <- function(good = TRUE) {
  if (good) success(42)
  else failure("cannot find good answer")
}

# Downstream users are now encouraged to inspect the result for success
risky_result <- risky_fn(good = TRUE)
if (is_success(risky_result)) {
  print(paste0("The answer: ", value(risky_result)))
}
#> [1] "The answer: 42"

risky_result <- risky_fn(good = FALSE)
if (is_failure(risky_result)) {
  cat(paste0("Failed because ", value(risky_result)))
}
#> Failed because cannot find good answer

Smart Results

If you are using others’ functions, wrap their results safely for immediate inspection, or wrap the whole function for safe use later:

# A third-party function that might crash with error
external_api <- function(good = TRUE) {
  if (good) 42
  else stop("Cannot connect to API")
}

# We can either safely inspect the results:
api_result <- as_result(external_api(good = TRUE))
is_success(api_result)
#> [1] TRUE
value(api_result)
#> [1] 42

# Or we can wrap the risky function for safe use later!
safely_call_api <- result(external_api)
safely_call_api(good = FALSE) |> is_failure()
#> [1] TRUE

Pipelines

We can chain together functions to create a pipeline that will either complete successfully or else stop gracefully at the first sign of failure.

Operations for our pipeline:

# Start by wrapping the first operation in a result to evaluate later
safely_call_api <- result(external_api)

# Other (risky) operations we want to safely use
add_some <- function(x, y) x + y
times_some <- function(x, y) x * y
times_too_much <- function(x, y) {
  z <- x * y
  if (z > 100) stop("Result has become too big") else z
}

A successful pipeline binding result operations together:

process <-
  safely_call_api(good = TRUE) |>
  then_try(times_some, 2) |>
  then_try(add_some, 10)

if (is_success(process)) {
  print(paste0("Processed: ", value(process)))
} else {
  cat(paste0("Could not process: ", value(process)))
}
#> [1] "Processed: 94"

A failing pipeline that should fail gracefully at the second step (times_too_much):

process <-
  safely_call_api(good = TRUE) |>
  then_try(times_too_much, 50) |>
  then_try(add_some, 10)

if (is_success(process)) {
  print(paste0("Processed: ", value(process)))
} else {
  cat(paste0("Could not process: ", value(process)))
}
#> Could not process: Result has become too big

Installation

You can install the released version of result from CRAN:

install.packages("result")

You can install the development version of result from GitHub with:

# install.packages("devtools")
devtools::install_github("soumyaray/result")

Why not Either or Maybe?

Result is a generalization of the Maybe type that is available to R devs from the maybe package. Conversely, the Maybe type is a special case of the Result type where the error type is Nothing. Maybe shines in situations when an operation might return something or nothing. But Result can convey more information than Maybe in error situations (e.g., a status code or message explaining the error). In this R implementation, the result() and as_result() functions also capture errors automatically, sparing developers from having to wrap expressions in their own tryCatch blocks.

The Result type is a special case of the Either type. Whereas Either resolves to Left or Right, Result usually resolves to notions of “Ok’ or”Error”. Use of Left and Right can be confusing to newcomers to functional concepts, and do not express the relevance of outcomes. In this R implementation of Result, the result() and as_result() functions resolve to success and failure respectively, which should be intuitive to many developers. Implementations of Result in other languages (such as Rust) sometimes use ok and error (or err), but these can conflict with variable names and keywords in other languages.