(Often a string with the error message, a pair with an error code and a message, or when retrofitted to a language with exceptions, an exception type). Only the "happy" type is a parameter, the other type is fixed to be some sort of type that represents an error.a list of characters, whereas Text is a more modern encoding-aware type.] Return an Error typeĪn Error type is similar to an Either type in that it represents the concept of returning either one type or another type. ![]() C# like this: Either ParseDecimal(string text) If you aren't used to reading Haskell type signatures, this could by roughly translated into e.g. ![]() So, it is a function that takes text as input and returns either a string (containing an error message) or a pair of an integer and the remaining text to be parsed. When we look at the type of the decimal function and unpack it a little bit (and assume a concrete type for the Integral a type constraint), we can see that it basically has type decimal :: Text -> Either String (Int, Text) It can be used for many different things, but it is often used to return either an intended result or a description of what went wrong.Īn example is the function in the Haskell text package. Return an Either typeĪn Either type, as the name implies, is a parametric type constructor that represents the concept of "either this type or that type". Typically, there is a getOrElse(default) method that either returns the value or a default value of your choosing. Then you need to extract it from the Option and decide what to do if it is not there. And you can ignore the " Optionness" most of the way, until you actually need the value. If there is a None at any step of the calculation, it will just be propagated, but it will never fail. Simply do for (const i of tryParse("hello")) console.log(i) So, instead of const i = parseInt("hello") print it)? Well, just iterate over the "collection": if it is empty, nothing will happen, and if the value exists, the body of the iteration will be executed exactly once. Collection operations are some of the most important operations in programming, so it is likely you are already proficient in them, and you can apply everything you know to Options as well.įor example, you want to execute a side-effect with the value if it exists (e.g. Option types have some really nice properties: they are not only monads, they are also collections. Here is an example from the Scala standard library: returns an Option. ![]() In fact, an Option type is isomorphic to a collection with at most one element, which means that it can implement all the collection APIs and thus be used like any other collection. You can also think of it as a collection of length "at most one". This is typically called an Option or Maybe type. Many languages / standard libraries have a type that represents the concept of "a value that may or may not be there" (or "zero or one of some thing"). Some common approaches to deal with this problem are: Return an optional type for parseInt there are countably infinite valid inputs and countably infinite invalid inputs.) ![]() There are typically at least as many invalid inputs as there are valid ones. Parsing functions are a complex beast, because they must be designed to fail.
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