Adding an extension method to a record with unit of measure

Question

Why does this work:

type Money = 
   { Amount : decimal } with

   member inline m.gotMoney : bool =
      m.Amount > 0M

but this doesn't

type MoneyUOM<[<Measure>]'currency> = 
   { Amount : decimal<'currency> } with

   member inline m.gotMoney : bool =
      m.Amount > 0M<_>

Instead, i get error FS0339: The signature and implementation are not compatible because the type parameter in the class/signature has a different compile-time requirement to the one in the member/implementation

Unfortunately, it's because you're using `decimal` instead of `float`. The issue is that `0M<_>` is not considered a generic value, while `0.0<_>` is. — kvb, Sep 11 '16 at 21:38
@kvb This answers the "why?" part of the question, in a way. Please convert it to an answer so you could get a well-deserved up-vote. — GregC, Sep 11 '16 at 23:46

score 5 · Accepted Answer · answered Sep 11 '16 at 23:23

DecimalWithMeasure is useful here. For example, this works for me:

type MoneyUOM<[<Measure>]'currency> = 
   { Amount : decimal<'currency> } with

   member m.gotMoney() : bool =
      let zero = LanguagePrimitives.DecimalWithMeasure<'currency> 0M
      m.Amount > zero

Adding an extension method to a record with unit of measure

1 Answers1