Re: [Caml-list] Annotated trees

[Martin Jambon <martin.jambon@ens-lyon.org>]

Martin Jambon wrote:
> Daniel Bünzli wrote:
>> Hello,
>>
>> Does anybody have any real world experience in choosing between the
>> following two representations for a tree augmented with annotations ?
>>
>> type 'a t = 'a * [ `Leaf of string | `Node of 'a t list ]
>> type 'a t = [ `Leaf of 'a * string | `Node of 'a * 'a t list ]
>>
>> Which one is more convenient to process, pattern match on, makes code
>> more readable etc. ?
> 
> I normally use the second form.
> 
> I see the following advantages over the tuple form:
> 
> 1. Pattern-matching is more readable because the most important piece of
> information comes first:
> 
>    `Leaf (_, s) -> ...
>  | `Node (_, l) -> ...
> 
> instead of:
> 
>    (_, `Leaf s) -> ...
>  | (_, `Node l) -> ...
> 
> 
> 2. If needed, writing an annotation_of_t function instead of just using fst is
> simple enough and not invasive.
> 
> 3. It is possible to not annotate certain kinds of nodes, or to have different
> types of annotations depending on the kind of node.
> 
> 4. The tuple version feels like there are 2 different definitions of a tree
> node, i.e. it is easy to get confused about whether an annotated node (the
> pair) or an non-annotated node (second member of the pair) is expected in one
> given place.
> 
> 5. I got this habit and therefore I won't change my mind and reject all
> rational arguments against it ;-)

See, I even did not consider the possibility of using:

type 'a t = [ `Leaf of string | `Node of 'a t list ] * 'a


Note that point (4) remains valid and based on real experience.



Martin

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