Why can't I pass arguments with units to F# types?

Question

Suppose I have defined a unit of measure:

[<Measure>] type Blob

And I want a type that takes a value in Blobs per second as a constructor argument:

type Something(flowRate:double<Blob/s>) = ...

F# throws a wobbly on this - "double does not expect any type arguments, but here is given 1 type argument"

I understand the message. I'd have thought it was obvious what I was trying to do, though I acknowledge that the syntax probably is verifiably wrong. Question is, how do I express this relationship in code?

Answer 1

As the message (sort of) indicates, doubles aren't measure-generic. Try float<Blob/s> instead. It's a bit strange, since float is a type synonym for type double . However, type float<[<Measure>]'t> is in some ways its own separate type. A similar problem occurs with single vs. float32 , int32 vs. int , and int8 vs. byte . Section 9.7 of the spec partly covers this information.

It's especially confusing since you can't define your own types which differ only in their measure arity:

type T = class end
type T<[<Measure>]'t> = class end //' Duplicate definition of type T