What are standard Scala monads other than Option?

Question

Option monad turns pretty much a facility some times in Scala. Are there any other monads in Scala standard library?

Answer 1

Luigi's answer is correct, but not very informative, IMHO.

All collections can implement the monad interface, but the signature for flatMap in them is not a monad's flatMap. They'll act like monads most of the time, though. Almost all of the classes listed by Luigi are related to collections.

LeftProject and RightProject refers to Either. Basically, Either is not a monad, but if you "project" one of the sides, then that side acts pretty much like an Option monad.

Parser is a monad, which forms the basis of parser combinators.

I admit I do not recognize ControlContext. I wonder if it is related to continuations (which are monads as well).

Answer 2

You can work this out by looking up the tell-tale flatMap in the API index. It gives:

FilterMonadic 
Stream 
StreamWithFilter 
TraversableMethods 
Iterator 
ParIterableLike 
ParIterableLike 
ParIterableViewLike 
TraversableLike 
WithFilter 
MonadOps 
TraversableProxyLike 
TraversableViewLike 
LeftProjection 
RightProjection 
Option 
WithFilter 
Responder 
Zipped 
ControlContext
Parser 

Answer 3

Here are the links to three sources files from Scalaz:

• Pure.scala
• Bind.scala
• Monad.scala

Take a look at instance declarations. That might give you an idea about what types from the standard library satisfy the monadic interface.