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Type constraint mismatch error
f#

I use the Powerpack matrix class. It is defining an operator * like

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    /// Multiply each element of a matrix by the given scalar value
    static member ( * ) : Matrix<'T> * 'T -> Matrix<'T>

However, I cannot use it in a generic function: 

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// this works, I can use '*' if type is fixed, like int 
let mm = Matrix.Generic.ofSeq [[ 1; 2; 3];[ 4; 5; 6];[ 7; 8; 9]]

let mmm = mm * 2

// Error 1	Type constraint mismatch when applying the default type 'Matrix<'T>' for a type inference variable. 

// The resulting type would be infinite when unifying ''T' and 'Matrix<'T>' Consider adding further type constraints	

let f2 (a:Matrix<'T>) (z:'T) = a*z

I do not understand the error message of the compiler. What does it mean and how do I fix it?

.

Unfortunately, it is not easy to write truly generic numeric code on .NET, because you cannot express operator constraints easily (e.g. 'T or Matrix<'T> must be a type that implements operator *). In case of your simple example however, you can use static member constraints:

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let inline f m x = m * x
// inferred type signature: ^a ->  ^b ->  ^c

//       when ( ^a or  ^b) : (static member ( * ) :  ^a *  ^b ->  ^c)


As I understand it, the effect of this is, that a call to f is replaced by a type-specialized version of the right hand side (at compile-time).

By on 6/26/2011 6:40 AM ()

Thanks very much for the reply. I tried it out. The following definition with type constraints compiles and dispatches to the correct function: 

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let inline f2 (m:^a) (x:^b) : ^c when (^a or ^b) : (static member (*) : ^a * ^b -> ^c) =
      m * x
By on 6/26/2011 12:28 PM ()

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