Taking the idea of phantom types explained in this post Implementing phantom types in F# , I'm trying to constrain the input parameter of an abstract type member, and more generally any type member. This may be impossible, and I'm happy to get that message.
Here is what I am attempting:
// my phantom type type Ascending = interface end type Descending = interface end type HeapOrder = inherit Ascending inherit Descending // my type type IHeap<'a when 'a : comparison> = inherit System.Collections.IEnumerable inherit System.Collections.Generic.IEnumerable<'a> ... abstract member Order : HeapOrder with get ... // ...and one or the other of these inherited types : type IHeap<'c, 'a when 'c :> IHeap<'c, 'a> and 'a : comparison> = inherit IHeap<'a> // or: type IHeap<'a, 'd when 'a : comparison and 'd :> HeapOrder> = inherit System.Collections.IEnumerable inherit System.Collections.Generic.IEnumerable<'a> type IHeap<'c, 'a, 'd when 'c :> IHeap<'c, 'a, 'd> and 'a : comparison and 'd :> HeapOrder> = inherit IHeap<'a> ... // ...and the member I want to constrain, so that the HeapOrder of the // input parameter matches the HeapOrder of the instantiated type. // Conceptually one of these alternatives (none of these build, // but they should convey what I'm attempting): // abstract member Merge : ('c when 'c<'c, 'a> : (member Order when Order :> 'd)) // or abstract member Merge : ('c when c' :> IHeap<'c, 'a, 'd>) -> 'c // or abstract member Merge : ('c when c' : #IHeap<'c, 'a, 'd>) -> 'c 
