monex v0.1.10 MonEx
MonEx implements two most common monadic data types:
MonEx.Result
- container for a result of operation or error. Result can be created using a constructor macro:ok(value)
orerror(e)
, where underlying structure is a tuple:{:ok, value}
or{:error, e}
respectively.MonEx.Option
- container for a value that might be present or missing. Usesome(value)
to create Option with value andnone()
to create an empty Option. Mind the parentheses, they are important when using it with pattern matching.MonEx
- collection of utility functions to work with both of these types.
Result
Result type fits perfectly with idiomatic Erlang/Elixir return values.
When some library function returns either {:ok, val}
or {:error, err}
,
you can use functions provided by MonEx right away. The most typical example,
where MonEx shines, is a pipeline, where each operation can fail. Normally
this would be organized in a form of nested case expressions:
final = case op1(x) do
{:ok, res1} ->
case op2(res1) do
{:ok, res2} -> op3(res2)
{:error, e} -> {:error, e}
end
{:error, e} -> {:error, e}
end
With MonEx you can do the same using flat_map
operation:
final = op1(x) |> flat_map(&op2/1) |> flat_map(&op3/1)
Once any of the operations returns error(e)
, following operations
are skipped and the error is returned. You can either do something
based on pattern matching or provide a fallback (can be a function or a default value).
case final do
ok(value) -> IO.puts(value)
error(e) -> IO.puts("Oh, no, the error occured!")
end
final |> fallback(ok("No problem, I got it"))
Option
Option type wraps the value. If value is present, it’s some(value)
,
if it’s missing, none()
is used instead. With Option type, you can use the
same set of functions, such as map
, flat_map
, etc.
find_user(id)
|> map(&find_posts_by_user/1)
This will only request for posts if the user was found. Then content of some()
will be transformed into posts, or none()
will be returned.
See docs per Result and Option modules for details.
Summary
Functions
Applies function that returns monadic type itself to the content of the monadic type. This is useful in a chain of operations, where argument to the next op has to be unwrapped to proceed
Performs a calculation with the content of monadic container and returns the argument intact. Even though the convention says to return nothing (Unit) this one passes value along for convenience — this way we can perform more than one operation
Transforms the content of monadic type.
Function is applied only if it’s ok
or some
.
Otherwise value stays intact
Types
Functions
Applies function that returns monadic type itself to the content of the monadic type. This is useful in a chain of operations, where argument to the next op has to be unwrapped to proceed.
Example:
inverse = fn (x) ->
if x == 0 do
none()
else
some(1/x)
end
some(5) |> flat_map(f) == some(1/5)
some(0) |> flat_map(f) == none()
Performs a calculation with the content of monadic container and returns the argument intact. Even though the convention says to return nothing (Unit) this one passes value along for convenience — this way we can perform more than one operation.
some(5)
|> foreach(fn x -> IO.inspect(x) end)
|> foreach(fn x -> IO.inspect(2 * x) end)
This will print: 5 10