Why does this F# computation expression give a warning?
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开发者_开发问答 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)
I sent the question to fsbugs@microsoft.com, and it's a bug. They said it'll be an error in the next release.
(They responded almost immediately, on Thanksgiving no less - it just took me a while to put the info here.)
I'm surprised that this works. Section 6.10 of the spec doesn't mention anything about synthesizing ReturnFrom
when it's not specified. Is there a reason not to just put a member x.ReturnFrom v = v
on the builder? Or a member x.Return(v) = Success v
, so that you could end your tracer
block with return x
, which would be more traditional?
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