Within 80 columns (not counting the startiong comment.
Hope your mailer does not insert or remove line-breaks.
#lang racket
;; <NOT TELLING OTHERS> : state -> (listof state) ;; given a state, compute
all possible successor states that can be reached in one boat crossing
(define (compute-possible-states state)
(local
[;; how-many : symbol bl -> number
;; to find out how many s are in a bl
(define (how-many s bl)
(cond
[(empty? bl) 0]
[else
(cond
[(symbol=? s (first bl))
(+ 1 (how-many s (rest bl)))]
[else (how-many s (rest bl))])]))
;; possible-states accumulate all the possible (an many incorrect)
;; future boat loads, given the current state.
(define (compute-a boat-loads possible-states)
(cond
[(empty? boat-loads) possible-states]
[else
(local
[(define num-missionaries (how-many 'M (first boat-loads)))
(define num-cannibals (how-many 'C (first boat-loads)))
(define (new-state op1 op2 boat-posn)
(make-state
(op1 (state-ml state)
num-missionaries)
(op1 (state-cl state)
num-cannibals)
(op2 (state-mr state)
num-missionaries)
(op2 (state-cr state)
num-cannibals)
boat-posn))
(define logical-boat-move
(cond
[(symbol=? (state-boat-posn state) 'left)
(new-state - + 'right)]
[else (new-state + - 'left)]))]
(compute-a (rest boat-loads)
(cons logical-boat-move possible-states)))]))]
(compute-a BOAT-LOADS empty)))
Jos
> -----Original Message-----
> From: users-boun...@racket-lang.org
> [mailto:users-boun...@racket-lang.org] On Behalf Of Horace Dynamite
> Sent: 02 August 2010 16:14
> To: Marco Morazan; users@racket-lang.org
> Subject: Re: [racket] syntax, differently
>
> On Mon, Aug 2, 2010 at 2:02 PM, Marco Morazan
> <moraz...@gmail.com> wrote:
> > Perhaps, you should consider making separate functions for
> each nested
> > cond and nested local. There is nothing "easy to read"
> about code with
> > the shape above. One cond and one local per function is a
> good rule of
> > thumb.
>
> Point taken, perhaps that was a poor example. Let me make
> clear that I wasn't trying to say that I ran into this
> 80-column "limit" often working HtDP, but there were
> occasional times when I felt it was necessary to break this.
> I've copied one such instance from a HtDP exercise below.
>
> > This the is the negative side of this, but the positive is
> that there
> > are lots of things that computers can help people do when
> it comes to
> > learning (eg, executing your program at all!). We should do
> our best
> > to exploit the computer's power to deal with mundane things
> (instead
> > of putting that burden on instructors/students).
>
> I do bow down to the people here having a lot more experience
> with these issues than I have, but I just find this statement
> a little weird with respect to the whole line-length limit
> debacle. As well as program execution, DrRacket's Stepper in
> the early teaching languages was so helpful to me. Of course
> I wouldn't want to do a hand-evaluation in favor of freeing
> the burden on the computer.
>
> But if something pisses you of when you're using the Stepper
> for example, thats because you don't know why you're program
> is wrong (e.g.), and now you have to go develop a headache
> figuring out whats the matter. Whereas if the "auto-magic
> line-breaker v 1.0" snaps your line when you really don't
> want it to, that pisses you off for the wrong reasons. It's
> an arbitrary decision made on your behalf because the
> computer thinks its smarter than it really is.
>
> I suppose what I'm butchering here in trying to say is that
> readers/compilers/debuggers for e.g., while they may be very
> hairy and complicated to implement, with more than one way to
> do them, they seem very rigid and omnipotent when you're
> using them as a novice. Whereas something like line-breaking
> seems very loose, personal and condition-dependent. Perhaps
> as a comprise for students who do listen, (and in my class in
> the UK, most students do! I'm by no means the best at all re.
> Matthias' comment) have the line breaker default if you must
> have it all, but on the first couple of runs, there should be
> a very obvious way to tell it where to go.
>
> Here is a part of my solution to an exercise in HtDP that
> breaks the line limit, and I don't think I'm being overly
> stupid in how I've done it,
>
> ;; <NOT TELLING OTHERS> : state -> (listof state) ;; given a
> state, compute all possible successor states that can be
> reached in ;; one boat crossing (define
> (compute-possible-states state)
> (local [;; how-many : symbol bl -> number
> ;; to find out how many s are in a bl
> (define (how-many s bl)
> (cond [(empty? bl) 0]
> [else (cond [(symbol=? s (first bl))
> (+ 1 (how-many s (rest bl)))]
> [else (how-many s (rest bl))])]))
> ;; possible-states accumulate all the possible (an many
> incorrect) future
> ;; boat loads, given the current state.
> (define (compute-a boat-loads possible-states)
> (cond [(empty? boat-loads) possible-states]
> [else (local [(define num-missionaries
> (how-many 'M (first boat-loads)))
> (define num-cannibals
> (how-many 'C (first boat-loads)))
> (define (new-state op1 op2 boat-posn)
> (make-state (op1 (state-ml state)
> num-missionaries)
> (op1 (state-cl state)
> num-cannibals)
> (op2 (state-mr state)
> num-missionaries)
> (op2 (state-cr state)
> num-cannibals)
> boat-posn))
> (define logical-boat-move
> (cond [(symbol=? (state-boat-posn
> state) 'left)
> (new-state - + 'right)]
> [else (new-state + -
> 'left)]))]
> (compute-a (rest boat-loads)
> (cons logical-boat-move
> possible-states)))]))]
> (compute-a BOAT-LOADS empty)))
>
> The longest line here is 90 characters, and breaking it would
> be very annoying. Further, this pattern isn't wacky,
> considering I'd just read the accumulation section in HtDP.
>
> Horace.
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