Real World Haskell by John Goerzen Bryan O’Sullivan & Donald Bruce Stewart

Author:John Goerzen Bryan O’Sullivan & Donald Bruce Stewart [Bryan O’Sullivan, John Goerzen]
Language: eng
Format: mobi
Tags: COMPUTERS / Programming Languages / General
ISBN: 9780596156923
Publisher: O'Reilly Media, Inc.
Published: 2008-11-15T05:00:00+00:00

Controlled Escape

The Monad typeclass doesn’t provide any means for values to escape their monadic shackles. We can inject a value into a monad using return. We can extract a value from a monad using (>>=) but the function on the right, which can see an unwrapped value, has to wrap its own result back up again.

Most monads have one or more runLogger-like functions. The notable exception is of course IO, which we usually escape from simply by exiting a program.

A monad execution function runs the code inside the monad and unwraps its result. Such functions are usually the only means provided for a value to escape from its monadic wrapper. The author of a monad thus has complete control over how whatever happens inside the monad gets out.

Some monads have several execution functions. In our case, we can imagine a few alternatives to runLogger: one might return only the log messages, whereas another might return just the result and drop the log messages.

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