[plt-scheme] idiomatic way to split a list into first, middle, last?

Sorry I read your code backwards -- Matthias


On Jan 6, 2006, at 2:50 AM, Bill Wood wrote:

> On Thu, 2006-01-05 at 23:44 -0500, Matthias Felleisen wrote:
>    . . .
>> You got the constructor wrong here. This wouldn't do it.
>
> I'm not sure what you mean by "constructor" here.  I gathered from
> reading the code in the original message that the intent was to extract
> the first item, the last item, and the list of intervening items from 
> an
> input list, and to apply first-proc to the first, middle-proc to each 
> of
> the intervening ones, and last-proc to the last.  I decided the nubbin
> of the problem was the extraction; applying the procs is trivial.  
> Hence
> I focussed on pattern-matching to extract the relevant parts of the
> input list.  The following is a transcript of an SWI Prolog session:
> ====================================================================
> $ pl
> Welcome to SWI-Prolog (Multi-threaded, Version 5.4.7)
> Copyright (c) 1990-2003 University of Amsterdam.
> SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
> and you are welcome to redistribute it under certain conditions.
> Please visit http://www.swi-prolog.org for details.
>
> For help, use ?- help(Topic). or ?- apropos(Word).
>
> ?- [user].
> |: trisect( List, First, Middle, Last ) :-
> |:   append( [First], Xs, List ),
> |:   append( Middle, [Last], Xs ).
> |: % user://1 compiled 0.00 sec, 424 bytes
>
> Yes
> ?- trisect( [1,2,3,4,5], F, M, L ).
>
> F = 1
> M = [2, 3, 4]
> L = 5 ;
>
> No
> ?- trisect( [1,2], F, M, L ).
>
> F = 1
> M = []
> L = 2 ;
>
> No
> ?-
> ====================================================================
>
> I think the variables F,M,L are bound correctly in both cases above.
>
>  -- Bill Wood
>
> PS Some notes for those who aren't familiar with Prolog.
>
> 1) Code is loaded into the interpreter by "consult"ing a file, and an
>    abbreviation for "consult( foo )" is "[ foo ].  "user" is initially
>    bound to stdin, so "[user]." at the prompt allows interactive entry
>    of code to the interpreter.  Oh yeah, each line entered at top level
>    must end with "." so the interpreter will know you're done.
>
> 2) In Prolog, variables are indicated by initial caps (or initial "_").
>
> 3) A definition of a predicate looks like
>       foo( A1,...An ) :- bar1( B11,...B1n1), ..., barm( Bm1,...Bmnm ).
>    Read this as "foo(...) succeeds if bindings for the variables in 
> each
>    of bar1,...,barm can be found which satisfy each of bar1(...) 
> through
>    barm(...)
>
>    So, the three lines of code following "|:" prompts in the transcript
>    define the predicate "trisect( List, First, Middle, Last )" to be
>    satisfied by bindings to the four variables List, First, Middle, 
> Last
>    if they satisfy both the subgoals "append( [First], Xs, List )" and
>    "append( Middle, [Last], Xs )"
>
> 4) "append( Xs, Ys, Zs )" is a built-in predicate that is read as "Zs 
> is
>    the result of appending the list bound to Xs to the list bound to Ys
>    (in that order).  If you want to append two lists, say [1,2,3] and
>    [7,6,5], and bind the result to Z, the call would be
>        append( [1,2,3], [7,6,5], Z )
>    and Z would be bound to the list [1,2,3,7,6,5].
>
>    On the other hand, if you want to decompose [1,2,3,7,6,5] into a 
> left
>    hand part and a right hand part bound to the variables Left and 
> Right
>    respectively, then the call would be
>       append( Left, Right, [1,2,3,7,6,5] )
>
> 5) The Prolog engine works by making choices of goals and, if 
> necessary,
>    backtracking through the tree of choice points trying all possible
>    paths.
>
>    At top level, when a goal succeeds the interpreter prints out the 
> the
>    successful bindings to all the variables in the goal, and then
>    pauses.  If you press <RETURN> you get "Yes" followed by the prompt;
>    if you press ";" followed by <RETURN> the interpreter fails the
>    current goal, so backtracking is attempted and, if successful, the
>    new variable bindings for the new successful goal are printed.  In
>    general, if a top-level goal fails the interpreter prints "No", so 
> if
>    you keep asking for alternatives eventually the goal fails, "No" is
>    printed followed by the prompt.
>
>    The transcript shows my request for all alternative sets of bindings
>    to the variables; in each call there was exactly one successful
>    binding set.
>
> 6) A list literal is written "[x1,...,xn]".
>
>  -- wtw
>
>