问题
This is a follow-up question to this question.
I'm trying to create a computation expression builder that accumulates a value through custom operations, and also supports standard F# language constructs at the same time. For the purposes of having a simple example to talk about, I'm using a computation expression that builds F# lists. Thanks to suggestions from kvb and Daniel I'm further along, but still having trouble with for
loops.
The builder:
type Items<'a> = Items of 'a list
type ListBuilder() =
member x.Yield(vars) = Items [], vars
member x.Run(l,_) = l
member x.Zero() = Items [], ()
member x.Delay f = f()
member x.ReturnFrom f = f
member x.Combine((Items curLeft, _), (Items curRight, vars)) =
(Items (curLeft @ curRight), vars)
member x.Bind(m: Items<'a> * 'v, f: 'v -> Items<'a> * 'o) : Items<'a> * 'o =
let (Items current, vals) = m
x.Combine(m, f vals)
member x.While(guard, body) =
if not (guard()) then
x.Zero()
else
x.Bind(body, fun () -> x.While(guard, body))
member x.TryWith(body, handler) =
try
x.ReturnFrom(body())
with e ->
handler e
member x.TryFinally(body, compensation) =
try
x.ReturnFrom(body())
finally
compensation()
member x.Using(disposable:#System.IDisposable, body) =
let body' = fun() -> body disposable
x.TryFinally(body', fun () ->
match disposable with
| null -> ()
| disp -> disp.Dispose())
member x.For(xs:seq<'a>, body) =
x.Using(xs.GetEnumerator(), fun enum ->
x.While(enum.MoveNext, x.Delay(fun () -> body enum.Current)))
[<CustomOperation("add", MaintainsVariableSpace=true)>]
member x.Add((Items current, vars), [<ProjectionParameter>] f) =
Items (current @ [f vars]), vars
[<CustomOperation("addMany", MaintainsVariableSpace=true)>]
member x.AddMany((Items current, vars), [<ProjectionParameter>] f) =
Items (current @ f vars), vars
let listBuilder = ListBuilder()
let build (Items items) = items
This version allows for things I could not do before, such as:
let stuff =
listBuilder {
let x = 5 * 47
printfn "hey"
add x
addMany [x .. x + 10]
} |> build
However, I'm still getting a compiler error on this one:
let stuff2 =
listBuilder {
for x in 1 .. 50 do
add x
} |> build
In this case, the IDE is underlining the x in for x in
and telling me, "This expression was expected to have type unit, but here has type int."
It's not really clear to me why it's expecting the loop variable to be of type unit. Clearly I've got the wrong method signature somewhere, and I suspect I'm not passing through my accumulated state in every place I should be, but the compiler error is really not helping me narrow down where I went wrong. Any suggestions would be appreciated.
回答1:
The immediate cause is that your While
function constrains the type of body
. However, in general you can't use both custom operations and also control flow operators in the same computation expression, so I don't think you'll ever be able to do exactly what you want even if you fix the signature.
来源:https://stackoverflow.com/questions/23144744/why-does-this-computation-expression-builder-expect-unit-in-my-for-loop