[Haskell-cafe] |> vs. $ (was: request for code review)
Shannon -jj Behrens
jjinux at gmail.com
Thu Mar 9 02:26:10 EST 2006
On 3/8/06, Jared Updike <jupdike at gmail.com> wrote:
> > 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.
Dude, that was a friggin' awesome email! I'm trying to figure out how
I can just copy it wholesale into the article ;) I've been struggling
with Haskell for long enough that my knowledge is now snowballing
downhill. Everything you said made sense 100%.
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