Re:The Early History of F# [pdf]
by choeger on Saturday June 13, 2020 @06:30AM
(#23508450)
> how do you do it actually for nontrivial preconditions like "this list must be sorted"?
This depends on your data model, of course. But for a list of integers you could do the following:
1. Have a unique data type that is the list of Deltas, plus the initial element (So for instance the list [5, 3, 6] would be encoded as (3, [2, 1]).
2. Provide a sort function that creates such a sorted list.
3. Make the sorted list your input parameter.
This is obviously oversimplified, but I hope you get the idea.
A cheaper alternative is to
1. Make an opaque datatype "sorted list", together with a function that translates this type to a normal list. Implemen this type just as a list, but keep the implementation private.
2. Provide a function sort, that is the only function that can yield that type.
This depends on your data model, of course. But for a list of integers you could do the following:
1. Have a unique data type that is the list of Deltas, plus the initial element (So for instance the list [5, 3, 6] would be encoded as (3, [2, 1]).
2. Provide a sort function that creates such a sorted list.
3. Make the sorted list your input parameter.
This is obviously oversimplified, but I hope you get the idea.
A cheaper alternative is to
1. Make an opaque datatype "sorted list", together with a function that translates this type to a normal list. Implemen this type just as a list, but keep the implementation private.
2. Provide a function sort, that is the only function that can yield that type.
3. Demand that type as input.