"Alexander S. Usov" <A.S.Usov@KVI.nl> writes:

> Hi!
>
> Can anybody expalin me why in such code
>
> ------
> type lex_node = Letter of char * bool * lex_tree
> and  lex_tree = lex_node list ;;
>
> exceprion Already ;;
>
> let rec insert lex word =
>   let whd = word.[0]
>   and wtl = String.sub word 1 (String.length word - 1)
>   in
>     try
>       match lex with
> 	| (Letter(c,b,t) as letter)::tail when (c <> whd) -> 
> 	    letter :: (insert tail word)
> 	| Letter(c,b,t) :: tail when (wtl = "") ->
> 	    if b = true then
> 	      raise Already
> 	    else
> 	      Letter(c,true,t) :: tail
> 	| Letter(c,b,t) :: tail ->
> 	    Letter(c,b,(insert t wtl)) :: tail
> 	| [] ->
> 	    if wtl = "" then
> 	      [ Letter (whd, true, []) ]
> 	    else
> 	      [ Letter (whd, false, insert [] wtl) ]
>     with
>       | Already -> lex ;;
>
> let a = insert [] "word"
> in
>   a == insert a "word" ;;
> -----
>
> returns false?

a #trace insert make it very clear to me : it's because the Already
exception is immediately caught. your code do exactly the same than :

let rec insert lex word =
   let whd = word.[0]
   and wtl = String.sub word 1 (String.length word - 1)
   in
      match lex with
	| (Letter(c,b,t) as letter)::tail when (c <> whd) -> 
	    letter :: (insert tail word)
	| Letter(c,b,t) :: tail when (wtl = "") ->
	    if b = true then
	      lex
	    else
	      Letter(c,true,t) :: tail
	| Letter(c,b,t) :: tail ->
	    Letter(c,b,(insert t wtl)) :: tail
	| [] ->
	    if wtl = "" then
	      [ Letter (whd, true, []) ]
	    else
	      [ Letter (whd, false, insert [] wtl) ]


you should catch the exception at the top most part of the insertion.

-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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