Cheatsheet: Option (in Rust) vs Maybe (in Haskell)

Correspondence of common combinators

This is meant to be for people coming from Haskell to Rust or vice versa who want to quickly find the name of corresponding function on optional values. For example, I keep forgetting the names of Rust combinators. Which one I have to use in a particular situation? Is it or_else, unwrap_or orunwrap_or_else? I can imagine that other people may experience similar problems, hence the cheatsheet. You can find examples and more detailed description in the official documentation by clicking on function names.

Update 1: as /u/jkachmar points out on Reddit that there is a note function in some of alternative Preludes in Haskell (or in errors package), which is an analog of ok_or in Rust. Added to the cheatsheet.

Update 2: /u/masklinn says that Haskell listToMaybe is akin to calling next on an Iterator and maybeToList is an Option implementing IntoIterator. I agree with that, since lists in Haskell, being lazy, more or less correspond to Rust iterators. Also, you can iterate over Option in Rust by calling iter.

Update 3: fixed several typos and errors spotted by Reddit readers. Thank you /u/jroller, /u/jlombera, /u/gabedamien, /u/george____t

Update 4: use flatten from Iterator to implement catMaybe, thanks to /u/MysteryManEusine.

Cheatsheet

Haskell

Rust

Purpose

Type (Haskell style)

Maybe

Option

type name

Just

Some

constructor for value

a -> Maybe a

Nothing

None

constructor for no value

Maybe a

isJust

is_some

check if has value

Maybe a -> Bool

isNothing

is_none

check if has no value

Maybe a -> Bool

fmap from Functor

map

apply function to value inside

(a -> b) -> Maybe a -> Maybe b

fromJust

unwrap

extract a value, fail if there is none

Maybe a -> a

fromMaybe

unwrap_or

extract a value or return a given default

a -> Maybe a -> a

(>>=) from Monad

and_then

propagate "no value", apply a function to a value, function can return no value too

Maybe a -> (a -> Maybe b) -> Maybe b

(<|>) from Alternative

or

return first value if present or second if not

Maybe a -> Maybe a -> Maybe a

(>>) from Monad

and

return first value if none or second if not

Maybe a -> Maybe b -> Maybe b

maybe

map_or

takes function and default. Apply function to the value or return default if there is no value

b -> (a -> b) -> Maybe a -> b

note, maybeToRight (both non-

standard)

ok_or

transforms optional value to possible error

b -> Maybe a -> Either b a

mapMaybe

filter_map from Iterator

applies filter and map simultaneously

(a -> Maybe b) -> [a] -> [b]

catMaybe

flatten from Iterator

extracts only values from the list, drops no values

[Maybe a] -> [a]

join from Monad

flatten

squashes two layers of optionality into one

Maybe (Maybe a) -> Maybe a

sequence from Traversable

transpose

transposes Option and Result layers (or Either and Maybe in Haskell terms)

Maybe (Either e a) -> Either e (Maybe a)

Notes

In Rust, all combinators with or at the end have a variant with or_else at the end: unwrap_or_else or or_else etc. Those variants take a closure for the default value and are lazily evaluated. They are recommended when you have a function call returning default value. In Haskell there is no need for this, since it is a lazy language by default.

In Rust there are many different ways for extracting the optional value:

  • unwrap which just fails if there is no value

  • unwrap_or which provides a default for that

  • unwrap_or_else where this default is calculated by a closure

  • unwrap_default where default is taken from Default trait implemented on the type

  • unwrap_none which fails if the value is not None and returns nothing

  • expect which is the same as unwrap, but takes a custom error message

  • expect_none which is the same as unwrap_none but takes a custom error message

Another very useful method in Rust is as_ref which converts from &Option<T> to Option<&T> which is very handy for pattern matching. as_mut plays a similar role for mutable references.

In Rust there are several methods related to ownership of the value in the Option, like take and replace, they have no analogs in Haskell that does not have the ownership concept.

In Rust we sometimes want to copy or clone values, it's possible to do so on optional references (Option<&T>) to get Option<T> from those by cloning or copying the referenced value. There are copied and cloned methods for this.

It's possible to mutate optional values in Rust. For that we have get_or_insert and get_or_insert_with methods which allow to insert a new (possibly computed) value into None or just use the value which was there.

transpose method in Rust is interesting, since it reminds me of sequence from Traversable in Haskell. It basically transpose two layers: Result (or Either in Haskell) and Option. sequence in Haskell does approximately the same, but in a more generic fashion.

In addition to and and or Rust has a method xor, perhaps just for the completeness. You've probably guessed that it returns Some if and only if there is only one Some in its arguments.

Haskell has two functions listToMaybe and maybeToList that convert between trivial lists (with 0 or 1 elements) and Maybe values. Rust doesn't have those, since lists are not that ubiquitous, but see the Update 2 above.

Summary

Rust has more functions to work with Option than Haskell because it has to support references, mutability and ownership. On the other hand Haskell outsources some of the combinators to its generic typeclasses: Semigroup, Alternative, Monoidetc. so its combinator library seems thinner.