[Haskell-cafe] Explaining monads

Ronald Guida ronguida at mindspring.com
Sun Aug 12 11:36:18 EDT 2007

Tillmann Rendel wrote:
 > Ronald Guida wrote:
 >> Here's a toy language, described by a regular expression:
 >>  0(10)*110
 >> I want to read characters, one at a time, and eventually decide to
 >> "Accept" or "Reject" a string.
 >> Let me try to understand my options.
 >> * With a simple Arrow, I can create a fixed sequence of "read"
 >>   operations, and I can't act on the results (i.e. by choosing
 >>   whether or not to keep reading) at run-time.
 > Nothing stops your Arrow-based RegExp-library from defining suitable
 > combinators to implement choices in RegExp's without using
 > ArrowChoice or ArrowApply. But if your Arrow-type is abstract, the
 > users of your library can only use the combinators you provided, so
 > you can safely assume they do only RegExp parsing, and optimize your
 > Arrows in the runRegExp-function for the special case of
 > RegExp-matching.

So it seems I was thinking too narrowly of arrows...

If I think of an arrow as defining a Domain Specific Embedded Language
(DSEL), then with a plain arrow, users can't embed Haskel inside the

 > But if you decide to expose ArrowApply-functionality to the users of
 > your RegExp-library, they are able to define arbitrary string
 > matching on top of your RegExp library, so you can't do any
 > optimizations because you never know what your users decided to do.

If I think of a monad (ArrowApply) as defining a Domain Specific
Embedded Language (DSEL), then users can embed anything they want
within the DSEL.

 > From a software engineering point of view, the idea of
 > Arrow-and-only-Arrow is to encode the whole computation in the
 > internal structure of the arrow, independent of the interpreting
 > language Haskell. This internal structure could be as expressible as
 > Haskell. In contrast, ArrowApply and Monad use regular Haskell
 > expressions for everything Haskell can do (like if-then-else,
 > recursion, ...) and only encode special operations into the internal
 > structure (like access to state, nondeterminism, ...).
 > This distinction is reflected in the treatment of variables in
 > arrow-based vs. monadic code. monadic code can use normal Haskell
 > variables, arrow-based code has to keep the variables "inside" the
 > arrow in some structure.

So if I want to explain arrows and monads as concisely as possible:

Arrows and monads are abstract data types used to construct Domain
Specific Embedded Languages (DSELs) within Haskel.  A simple arrow
provides a closed DSEL.  A monad is a special type of arrow that
creates an open DSEL by allowing users to embed arbitrary Haskel
within it.

-- Ron

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