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Exercise: ControlFlow-3

Many built-in functions have two forms. The xxx form returns the tuple {:ok, data} and the xxx! form returns data on success but raises an exception otherwise. However, some functions don't have the xxx! form.

Write an ok! function that takes an arbitrary parameter. If the parameter is the tuple {:ok, data}, return the data. Otherwise, raise an exception containing information from the parameter.

You could use your function like this:

file = ok! File.open("somefile")

Solution

I'm not going to bother putting this in a module. Let's just do this with an anonymous function. It can use pattern matching to identify whether or not {:ok, data} was passed in.

ok! = fn
  {:ok, data} -> data
  _ -> raise RuntimeError
end

In iex:

$ touch exists.txt
$ iex
iex> ok! = fn
...>   {:ok, data} -> data
...>   _ -> raise RuntimeError
...> end
#Function<6.50752066/1 in :erl_eval.expr/5>
iex> ok!.(File.open("exists.txt"))
#PID<0.64.0>
iex> ok!.(File.open("doesnt_exist.txt"))
** (RuntimeError) runtime error