I'm afraid this kind of 5-minute talk makes sense only if you already know a lot about monads or are a computer scientist; not if you're a programmer who wants to learn a new language. For instance, this statement starts making sense only if you've seen a lot of examples of monads (maybe even read Moggi and Wadler) and want to understand the big picture. "... when Haskell programmers want to perform a side effect, they explicitly construct a description of the side effecting computation as a value." And even then, what does it mean to "construct a description of the side effecting computation as a value" for the Maybe monad? An IO action or a State Monad action indeed are values that describe computations, but what computation does (Just 1) describe? It's the simple monads that are tricky to explain (I've seen a discussion to that effect in this forum and I wholeheartedly agree). --Bartosz On Sunday, September 16, 2012 7:49:51 AM UTC-7, Tillmann Rendel wrote:
Hi,
Kristopher Micinski wrote:
Everyone in the Haskell cafe probably has a secret dream to give the best "five minute monad talk."
(1) Most programming languages support side effects. There are different kinds of side effects such as accessing mutable variables, reading files, running in parallel, raising exceptions, nondeterministically returning more than one answer, and many more. Most languages have some of these effects built into their semantics, and do not support the others at all.
(2) Haskell is pure, so it doesn't support any side effects. Instead, when Haskell programmers want to perform a side effect, they explicitly construct a description of the side effecting computation as a value. For every group of related side effects, there is a Haskell type that describes computations that can have that group of side effects.
(3) Some of these types are built in, such as IO for accessing the world outside the processor and ST for accessing local mutable variables. Other such types are defined in Haskell libraries, such as for computations that can fail and for computations that can return multiple answers. Application programmers often define their own types for the side effects they need to describe, tailoring the language to their needs.
(4) All computation types have a common interface for operations that are independent of the exact side effects performed. Some functions work with arbitrary computations, just using this interface. For example, we can compose a computation with itself in order to run it twice. Such generic operations are highly reusable.
(5) The common interface for constructing computations is called "Monad". It is inspired by the mathematical theory that some computer scientists use when they describe what exactly the semantics of a programming language with side effects is. So most other languages support some monad natively without the programmer ever noticing, whereas Haskell programmers can choose (and even implement) exactly the monads they want. This makes Haskell a very good language for side effecting computation.
Tillmann
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