On 02.03.2009, at 01:50, Rich Hickey wrote:
> I was wondering if you considered using maps or struct-maps for this.
> One of my pet peeves with algebraic data types is the unnamed,
> positional nature of the components/fields. It always bothers me you
> have to rename the fields in every pattern match, skip fields with _
> etc.
There is something similar on my list of ideas for future extensions
of the algebraic data type module: optional keywords to name the
parameters of the constructors, which could then also be used for
matching in templates. Each component could then be referred to
either by its position or by its keyword. The model I have in mind is
function parameter passing in Python. The concrete representation
would still be vectors, probably, but no one should rely on the
concrete representation anyway. At the moment I use vectors because
template matching expands to code that uses nth for element access,
which is fastest for vectors.
I wouldn't like to have only named elements, because for constructors
with few arguments (one, at the extreme), this would be needlessly
verbose. Consider a very simple algebraic data type like Haskell's
maybe:
(deftype maybe
nothing
(just x))
Now that I re-think about this, it would make sense not to have
additional keywords as names, but use the constructor argument names
directly. All that would be required in addition to the current
mechanisms is an additional name-based template syntax.
BTW, another idea I have for future extensions is types with lazy
fields, with the constructors defined as macros and the component
values stored in delay objects. That would allow the construction of
lots of lazy data structures.
Konrad.
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