Yet Another Monad Tutorial

Phil Molyneux
Tue, 12 Aug 2003 15:10:09 +0100 (BST)

I think the crucial distinction is to think about is the difference
between calculating a value (such as a character) compared to calculating
an action (such as a computation that reads a character) --- one is wholly
in your programming language while the other interacts with something
outside your language (the I/O system). Many years ago we were all told it
was a *good thing* to separate calculating values from *doing* I/O. Of
course, you weren't actually *doing* any I/O when you *wrote* your program
--- you were *calculating* (working out, writing...) your program.

For most actions/computations, the properties you usually want is that A
followed by (B followed by C) is the same as (A followed by B) followed by
C and a unit (as in multiplication) or zero (as in addition) action. I
guess the usefulness of this is quite subtle --- the Romans had no
notation for zero and look what happened to them (although we still use
Roman numerals for decoration). How do you know you have structured your
program so that the preceding properties hold ? You show that you're
working in the appropriate Monad --- remember, maths is the QA of
programming. I suspect that tutorials should at some point mention some
definitions of monads, monoids et al --- since this is where the power
(sorry, QA) comes from. 


On Tue, 12 Aug 2003, Bayley, Alistair wrote:

> Date: Tue, 12 Aug 2003 12:10:24 +0100
> From: "Bayley, Alistair" <>
> Subject: RE: Yet Another Monad Tutorial
> > From: Wolfgang Jeltsch []
> > 
> > For example, the function readFile is pure. For a specific 
> > string s the 
> > expression readFile s always yields the same result: an I/O 
> > action which 
> > searches for a file named s, reads its content and takes this 
> > content as the 
> > result of the *action* (not the expression).
> What about getChar? This is a function which takes no arguments, yet returns
> a (potentially) different value each time. I know, I know: it returns an IO
> action which reads a single char from the terminal and returns it.
> Is the IO monad the only one (currently) in which you would say that it
> "returns an action", which is then executed by the runtime system? I would
> have thought that monads that are not involved in IO (e.g. State) would be
> pure in the sense that Van Roy was thinking. You wouldn't need to describe
> functions in them as "returning an action".
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Phil Molyneux                             email
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