Standard Library: Err

Throw and catch recoverable errors

The Err module allows programs to signal and respond to anticipated errors using the throw/try style control flow found in other programming languages.

  1. orElse
  2. throw
  3. try

Err.orElse(func, default)

Call the zero-argument function func. If func returns a value without throwing an error, return it; otherwise return default. 

fn inverse(n)
  if n == 0 then
    Err.throw("n must not be zero")
  end

  1 / n
end

Err.orElse(fn() inverse(5) end, none)
Err.orElse(fn() inverse(0) end, none)
0.2
none

Err.throw(payload)

Throw an error with a payload, which may be any type of value. Functions which throw errors should be called using try to avoid unhandled errors. 

fn inverse(n)
  if n == 0 then
    Err.throw("n must not be zero")
  end

  1 / n
end

inverse(0)
Error: unhandled error

Err.try(func, handler)

Call the zero-argument function func. If func returns a value without throwing an error, return it; otherwise, call handler with the error payload and return handler(payload). 

fn inverse(n)
  if n == 0 then
    Err.throw("n must not be zero")
  end

  1 / n
end

fn showErr(msg)
  "an error occured: $msg"
end

Err.try(fn() inverse(5) end, showErr)
Err.try(fn() inverse(0) end, showErr)
0.2
"an error occured: n must not be zero"