This code:
type Result = Success of string
type Tracer() =
member x.Bind(p: Result, rest: (string -> Result)) =
match p with
| Success s -> rest s
let tracer = new Tracer()
let t = tracer {
let! x = Success "yes!"
let! y = Success "waste of time"
return! Success x
}
printfn "%A" t
prints Success "yes!"
But gives a warning that implies that it shouldn't work:
File1.fs(19,3): warning FS0708: This control construct may only be used if the computation expression builder defines a 'ReturnFrom' method
Seems like a strange warning: if it's right, then the code shouldn't work. Is it really just saying that the builder had to synthesize ReturnFrom?
(F# Version 1.9.7.4, compiling for .NET Framework Version v4.0.21006)