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Introduction

The monads covered in Part II include a mixture of standard Haskell types that are monads as well as monad classes from Andy Gill's Monad Template Library. The Monad Template Library is included in the Glasgow Haskell Compiler's hierarchical libraries under Control.Monad

Some of the documentation for these monads comes from the excellent Haskell Wiki. In addition to the monads covered here, monads appear many other places in Haskell, such as the Parsec monadic combinator parsing library. These monads are beyond the scope of this reference, but they are thoroughly documented on their own. You can get a taste of the Parsec library by looking in the source code for example 16.

Monad Type of computation Combination strategy for >>=
Identity N/A — Used with monad transformers The bound function is applied to the input value.
Maybe Computations which may not return a result Nothing input gives Nothing output
Just x input uses x as input to the bound function.
Error Computations which can fail or throw exceptions Failure records information describing the failure. Binding passes failure information on without executing the bound function, or uses successful values as input to the bound function.
[] (List) Non-deterministic computations which can return multiple possible results Maps the bound function onto the input list and concatenates the resulting lists to get a list of all possible results from all possible inputs.
IO Computations which perform I/O Sequential execution of I/O actions in the order of binding.
State Computations which maintain state The bound function is applied to the input value to produce a state transition function which is applied to the input state.
Reader Computations which read from a shared environment The bound function is applied to the value of the input using the same environment.
Writer Computations which write data in addition to computing values Written data is maintained separately from values. The bound function is applied to the input value and anything it writes is appended to the write data stream.
Cont Computations which can be interrupted and restarted The bound function is inserted into the continuation chain.
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