[Haskell-cafe] Tutorial: Haskell for the Evil Genius

Kristopher Micinski krismicinski at gmail.com
Sun Sep 16 22:06:39 CEST 2012

Agreed.  Great.  I still contend that it would be cool to get this to
be a real thing at something like the Haskell workshop, I think
hearing the different perspectives would be an interesting insight
into the many different ways to explain monads.  But I suppose the way
to start would be to put up a webpage for collecting them..


On Sun, Sep 16, 2012 at 3:55 PM, Conal Elliott <conal at conal.net> wrote:
> Hi Tillmann. Wow. Lovely and spot on! And I almost never hear monad
> explanations without wincing. Thanks for sharing.  -- Conal
> On Sun, Sep 16, 2012 at 7:48 AM, Tillmann Rendel
> <rendel at informatik.uni-marburg.de> 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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