Issue with generic Unit of Measures / match











up vote
1
down vote

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I have read quite a few ressources on Unit of Measures (including the good Microsoft Doc and f# for fun and profit) but I still cannot make my code work.



[<Measure>] type USD

[<Measure>] type JPY

[<Measure>] type EUR



type FxUnit<[<Measure>] 'u,[<Measure>] 'v> = {UnitRate : float<'u/'v>}



let getFx1 u  =

    match u with

    | "USD" -> {UnitRate = 1.0<USD/USD>}

    | "EUR" -> {UnitRate = 1.0<USD/EUR>}


This way I get an error under:



1.0<USD/EUR>


I have tried thse:



let getFx1  u : (float<'u>) =

let getFx1  u : (float<'u/'v>) =


without much success. It seems that with a match, I can't return some type FxUnit with different UoM.



Any idea how I could fix this?



thanks.






f#







share|improve this question


















  • 1




    This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
    – dumetrulo
    Nov 20 at 12:50








  • 1




    The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
    – AMieres
    Nov 20 at 14:08















up vote
1
down vote

favorite












I have read quite a few ressources on Unit of Measures (including the good Microsoft Doc and f# for fun and profit) but I still cannot make my code work.



[<Measure>] type USD

[<Measure>] type JPY

[<Measure>] type EUR



type FxUnit<[<Measure>] 'u,[<Measure>] 'v> = {UnitRate : float<'u/'v>}



let getFx1 u  =

    match u with

    | "USD" -> {UnitRate = 1.0<USD/USD>}

    | "EUR" -> {UnitRate = 1.0<USD/EUR>}


This way I get an error under:



1.0<USD/EUR>


I have tried thse:



let getFx1  u : (float<'u>) =

let getFx1  u : (float<'u/'v>) =


without much success. It seems that with a match, I can't return some type FxUnit with different UoM.



Any idea how I could fix this?



thanks.






f#







share|improve this question


















  • 1




    This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
    – dumetrulo
    Nov 20 at 12:50








  • 1




    The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
    – AMieres
    Nov 20 at 14:08













up vote
1
down vote

favorite









up vote
1
down vote

favorite











I have read quite a few ressources on Unit of Measures (including the good Microsoft Doc and f# for fun and profit) but I still cannot make my code work.



[<Measure>] type USD

[<Measure>] type JPY

[<Measure>] type EUR



type FxUnit<[<Measure>] 'u,[<Measure>] 'v> = {UnitRate : float<'u/'v>}



let getFx1 u  =

    match u with

    | "USD" -> {UnitRate = 1.0<USD/USD>}

    | "EUR" -> {UnitRate = 1.0<USD/EUR>}


This way I get an error under:



1.0<USD/EUR>


I have tried thse:



let getFx1  u : (float<'u>) =

let getFx1  u : (float<'u/'v>) =


without much success. It seems that with a match, I can't return some type FxUnit with different UoM.



Any idea how I could fix this?



thanks.






f#







share|improve this question













I have read quite a few ressources on Unit of Measures (including the good Microsoft Doc and f# for fun and profit) but I still cannot make my code work.



[<Measure>] type USD

[<Measure>] type JPY

[<Measure>] type EUR



type FxUnit<[<Measure>] 'u,[<Measure>] 'v> = {UnitRate : float<'u/'v>}



let getFx1 u  =

    match u with

    | "USD" -> {UnitRate = 1.0<USD/USD>}

    | "EUR" -> {UnitRate = 1.0<USD/EUR>}


This way I get an error under:



1.0<USD/EUR>


I have tried thse:



let getFx1  u : (float<'u>) =

let getFx1  u : (float<'u/'v>) =


without much success. It seems that with a match, I can't return some type FxUnit with different UoM.



Any idea how I could fix this?



thanks.






f#




f#






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 20 at 11:05









Jeff_hk

736




736








  • 1




    This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
    – dumetrulo
    Nov 20 at 12:50








  • 1




    The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
    – AMieres
    Nov 20 at 14:08














  • 1




    This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
    – dumetrulo
    Nov 20 at 12:50








  • 1




    The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
    – AMieres
    Nov 20 at 14:08








1




1




This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
– dumetrulo
Nov 20 at 12:50






This seems to be consistent with the general usage of type parameters; for example, if you have a function returning a 'a list, you cannot return a float list from one branch and a int list from another branch. It might work if you declare your function getFx1 as inline (as the compiler will then use statically resolved type parameters).
– dumetrulo
Nov 20 at 12:50






1




1




The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
– AMieres
Nov 20 at 14:08




The thing with Unit of Measures is that they are a feature to let the compiler verify that your arithmetic logic is sound so you do not end up for instance adding Yens + Euros. For that to work you need to have values with specific UoMs. If they were to be generic then the compiler cannot help you anymore and they lose their usefulness. So, there are very few things you can do with generic UoMs.
– AMieres
Nov 20 at 14:08












1 Answer
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up vote
9
down vote



accepted










The problem is that you're trying to define a function that takes a string and returns a value of type either FxUnit<USD,USD> or FxUnit<USD,EUR>. A function can't have two different return types, decided at runtime.



You could make FxUnit take two values of a discriminated union instead of measures:



type Currency =

    | USD

    | JPY

    | EUR



type FxUnit = { UnitRate : float; From : Currency; To : Currency }



let getFx1 u  =

    match u with

    | "USD" -> {UnitRate = 1.0; From = USD; To = USD}

    | "EUR" -> {UnitRate = 1.0; From = USD; To = EUR}

    | _     -> failwithf "Unrecognised units %s" u





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    up vote
    9
    down vote



    accepted










    The problem is that you're trying to define a function that takes a string and returns a value of type either FxUnit<USD,USD> or FxUnit<USD,EUR>. A function can't have two different return types, decided at runtime.



    You could make FxUnit take two values of a discriminated union instead of measures:



    type Currency =
    
    | USD
    
    | JPY
    
    | EUR
    

    
type FxUnit = { UnitRate : float; From : Currency; To : Currency }
    

    
let getFx1 u  =
    
    match u with
    
    | "USD" -> {UnitRate = 1.0; From = USD; To = USD}
    
    | "EUR" -> {UnitRate = 1.0; From = USD; To = EUR}
    
    | _     -> failwithf "Unrecognised units %s" u





    share|improve this answer

























      up vote
      9
      down vote



      accepted










      The problem is that you're trying to define a function that takes a string and returns a value of type either FxUnit<USD,USD> or FxUnit<USD,EUR>. A function can't have two different return types, decided at runtime.



      You could make FxUnit take two values of a discriminated union instead of measures:



      type Currency =
      
    | USD
      
    | JPY
      
    | EUR
      

      
type FxUnit = { UnitRate : float; From : Currency; To : Currency }
      

      
let getFx1 u  =
      
    match u with
      
    | "USD" -> {UnitRate = 1.0; From = USD; To = USD}
      
    | "EUR" -> {UnitRate = 1.0; From = USD; To = EUR}
      
    | _     -> failwithf "Unrecognised units %s" u





      share|improve this answer























        up vote
        9
        down vote



        accepted







        up vote
        9
        down vote



        accepted






        The problem is that you're trying to define a function that takes a string and returns a value of type either FxUnit<USD,USD> or FxUnit<USD,EUR>. A function can't have two different return types, decided at runtime.



        You could make FxUnit take two values of a discriminated union instead of measures:



        type Currency =
        
    | USD
        
    | JPY
        
    | EUR
        

        
type FxUnit = { UnitRate : float; From : Currency; To : Currency }
        

        
let getFx1 u  =
        
    match u with
        
    | "USD" -> {UnitRate = 1.0; From = USD; To = USD}
        
    | "EUR" -> {UnitRate = 1.0; From = USD; To = EUR}
        
    | _     -> failwithf "Unrecognised units %s" u





        share|improve this answer












        The problem is that you're trying to define a function that takes a string and returns a value of type either FxUnit<USD,USD> or FxUnit<USD,EUR>. A function can't have two different return types, decided at runtime.



        You could make FxUnit take two values of a discriminated union instead of measures:



        type Currency =
        
    | USD
        
    | JPY
        
    | EUR
        

        
type FxUnit = { UnitRate : float; From : Currency; To : Currency }
        

        
let getFx1 u  =
        
    match u with
        
    | "USD" -> {UnitRate = 1.0; From = USD; To = USD}
        
    | "EUR" -> {UnitRate = 1.0; From = USD; To = EUR}
        
    | _     -> failwithf "Unrecognised units %s" u






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 20 at 13:14









        marklam

        4,80511620




        4,80511620






























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