Some programs fail because something is broken. Many programs simply do not find a value. A library card may not exist yet. A seat may be full. A receipt may not have a discount. Treating all of those as crashes makes ordinary work sound dangerous.
Option is for expected absence. It says, "there may be a value here." Result is for a recoverable success or failure. It says, "the work either produced a value or produced a reason it could not." Those are different stories, so Musi keeps them in different shapes.
let Option := import "@std/option";
let Result := import "@std/result";
let badge := Option.someOf[Int](42);
let emptyBadge := Option.noneOf[Int]();
let paid := Result.ok[Int, String](1200);
let declined := Result.err[Int, String]("card declined");
paid;The first value is like a coat-check ticket that found a coat. The second value is like a coat-check ticket with no coat attached yet. The result values are different: one says the payment produced a receipt number, and one says the payment failed with a message.
Option is not a quiet null
A quiet null makes every later line suspicious. Does this customer have an address? Did the search fail? Did somebody forget to fill a field? With Option, the type tells you that absence is part of the plan.
Use someOf when the value exists. Use noneOf when the empty case is expected. A pet shelter can have an optional adopter, a delivery can have an optional driver, and a room can have an optional booking. None of those cases should pretend to be a normal record with missing fields.
Result carries the reason
Use Result when the caller needs a reason. A payment might fail because the card was declined. A parse might fail because the text was not a number. A file load might fail because the path was wrong. The reason matters because the caller can choose a message, retry, or ask the user for a different value.
Do not use Result only because a value is absent. If a search for an open table returns nothing, Option is usually enough. If the reservation service could not be reached, Result tells the caller what went wrong.
How to choose
Ask a small question: would an empty answer be normal? If yes, use Option. Would the caller need to know why work failed? If yes, use Result. Did the code ask the outside world to do something? Then also read the effects chapters, because failure and outside work are related but not the same thing.