Type constraint mismatch when using List.sum

Question

I may have been away from F# for too long, but I would have expected this to compile:

module Calc

let foo values = List.sum values

Instead, I get a compilation error:

FS0071 Type constraint mismatch when applying the default type 'obj' for a type
inference variable. The type 'obj' does not support the operator '+' Consider
adding further type constraints

This also happens for List.average, so at first I thought it had something to do with generic constraints like requires member ( + ) and member get_Zero, but that can't be the whole story, since let foo values = List.max values compiles (List.max also has a generic constraint, albeit a simpler one - it only requires comparison).

What am I doing wrong?

This is a .NET 5.0 project. Compilation fails both in Visual Studio and with dotnet build.

Answer 1

List.sum has an SRTP type with a requirement that the element of the list has a (+) operator defined on it:

val sum:
   list: list< ^T>
      -> ^T (requires static member ( + ) and static member Zero )

When you try to call that without knowing the type of the list element, the compiler can't resolve the SRTP.

---

One option would be to mark foo itself inline:

let inline foo values = List.sum values

That way, foo itself will inherit the SRTP requirement:

val foo:
   values: list< ^a>
        -> ^a (requires static member ( + ) and static member Zero )

And you're just kicking the problem down the line - presumably to the point where you know the type of the element.

This doesn't work without an inline modifier, because SRTPs can't be compiled to IL.

---

Another option, if you can afford it, is to specify an element type, such that it has a (+) operator:

let foo (values: int list) = List.sum values

This way the compiler can resolve SRTP right there by observing that int does indeed have a (+) operator.

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This works fine with List.max, because max doesn't have an SRTP constraint on it. It only has an 'T :> IComparison constraint, which are supported by the IL, so they don't have to be SRTP.