F# container that implements equality only when underlying type does -


edit: appears answer , comment added far have not explained want. here example:

// type not supporting type of comparison [<noequality>] [<nocomparison>] type blah () =     member x.huha = 0  // make map, turns out work whether x supports equality or not let inline tt x =     map.oflist [1, x]         let test () =     // maps can compared equality if argument can     if (tt 1 = tt 2) failwithf "strange"      // maps can made no matter if argument supports equality     if (tt (blah ())).count <> 1 failwithf "size"      // not compile     if tt (blah ()) = tt (blah ()) ....

in short, want own type behave map above. should support equality when type argument does, , should not when type argument doesn't. want typechecker stop me using equality when not supported, seeing can builtin types. again.

original question: various built-in f# types support equality if , if underlying type does. example, map<'k, 'd> support equality iff 'd (and detected @ compile-time). possible implement behaviour in user-code? here 1 failed attempt, , version compiles fine if equality unconditional. many thanks.

[<nocomparison>] type test_fails<[<equalityconditionalon>]'a> (content:'a) =      let eq_impl (x:test_fails<'a>) (y:obj) =         let y = y :?> test_fails<'a>         x.content = y.content      member x.content = content      override x.equals (y:obj) =         eq_impl x y  [<nocomparison>] type test_compiles<'a when 'a : equality> (content:'a) =      let eq_impl (x:test_compiles<'a>) (y:obj) =         let y = y :?> test_compiles<'a>         x.content = y.content      member x.content = content      override x.equals (y:obj) =         eq_impl x y

you have part of solution already: using [<equalityconditionalon>] on generic parameter.

the part missing: need use unchecked.equals instead of normal = operator within map implementation (anywhere you're checking equality of 2 'a values). unchecked.equals checks @ run-time whether type supports generic equality. if does, compares 2 instances/values equality usual; if not, falls structural equality check or type's implementation of object.equals(obj) method.


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