[Haskell-cafe] |> vs. $ (was: request for code review)

Jared Updike jupdike at gmail.com
Wed Mar 8 20:52:11 EST 2006

> I suspect you guys are right.  I had always thought of states as
> being "isomorphic" to integers (i.e. you can be in state 0, state 1,
> ... state n), not as contexts (you have this input, that output, and
> this token stack), am I wrong?

You're thinking of a state machine, I think, which is not quite what a
state monad would do here. (I have nightmares of writing a
state-machine parser in assembly like I did in an EE class once...

>  I suspect I need to spend more time
> trying to understand the state monad.  I must admit that I baulked
> the last time I tried to squeeze it into my head.  I'll just need to
> try again ;)

Here's the way I like to think about state in imperative
programs---it's hard because it's not something you can get far away
from enough to see, usually.

In imperative programs, the value of a variable 'a' at one point is
not always the value of the variable 'a' at another point later in the
code. In some sense, each statement that gets executed is passed the
entire state of the machine (the world) implicitly, and then when the
statement ends, it passes the state of the world on to the next
statement. If you want to access the value of the variable 'a', then
'a' gets looked up in the environment/state.
In C/C++/Java/C#/Python/Perl, etc. this is done for you automatically
and efficiently. It's just the way the machine works. But you don't
have the choice to change this or, as someone put it, "overload the

In Haskell none of this variable-mutating, state-passing **can**
occur, so it gets created from scratch, and voila, we have the State
Monad. It makes it sound like a lot more work than it should be just
to do something that comes for free in most other languages, but in
these languages, you can't overload the semicolon! And if you could,
who knows what could go wrong at runtime (imagine Perl with semicolon
overloading... I bet some day they will do this just because they
can...). In Haskell, everything is watched over by the type system, so
the parts of your program that explicitly need to munge state are
isolated with the some type tag, e.g. ParseContext, while the rest of
your program is "normal" and pure and functional.

The problem with monads is not that they are "advanced" but that they
are so painfully and subtly abstract (I was going to say "subtly
simple" but maybe they aren't for most working non-Haskell
programmers...). (It just so happens that you **can** do amazing,
convenient, efficient, magic and otherwise advanced things with them,
especially with the libraries.) Another problem is that everyone has
different ways of explaining them or trying to define what they are (a
way of sequencing computation? or a type constructor? or a type
class?). Of course, they are all those things, which makes it even
more confusing. At a certain point, though, I think they just
subconciously click and boom, now you get it.

Anyway, if your goal is to get people to understand Haskell, then see
if you can use monads to simplify things. If your goal is to do a
straight translation of the C code, don't worry about monads.

My 2c,
reverse ")-:"

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