As an example, this...
- a
- b c
- d e f
- g h
would get transformed into something like this...
'(a (b c) (d e f (g h)))
You can implement this in 2 stages. Stage 1 is line based parsing, turn
the input text into this list:
'((0 a)
(2 b c)
(2 d e f)
(4 g h)))
the number is the indent level. This part is quite easy, you could do
this with regex.
Stage 2 is a bit more difficult, the algorithm to do this is based on
recursive stream processing.
You are reading or 'parsing' a tree off of a flat string of indent
levels.
When processing a head at indent level N you read off all the subtrees
at indent level >N you can see,
then return two values: the tree and the remainder of the stream.
(define (collect inputs)
(let-values (((h t) (collect^ inputs)))
(unless (null? t)
(error 'collect "failed to collect everything" t))
h))
(define (collect^ inputs)
(let ((indent (caar inputs))
(head (cdar inputs))
(inputs (cdr inputs)))
(n-collect indent head inputs)))
(define (n-collect n head stream)
;; n-collect will collect up all
;; subtrees off the stream whose
;; level is > N
(let loop ((subtrees '())
(stream stream))
(if (or (null? stream)
(<= (caar stream) n))
(values (append head (reverse subtrees)) stream)
(let-values (((subtree stream) (collect^ stream)))
(loop (cons subtree subtrees)
stream)))))
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