Troubleshooting Types

This page shows the compiler errors that usually mean you crossed a wrapper boundary in the wrong place.

Most FsFlow type errors are not exotic. The compiler usually sees one wrapper shape and you intended another.

Error: A Unique Overload For Method Bind Could Not Be Determined

This usually happens when the compiler cannot tell which wrapper shape a let! value should use.

Example:

let nested : Async<Async<Result<int, string>>> =
    async {
        return async { return Ok 42 }
    }

let workflow : Flow<unit, string, int> =
    flow {
        let! next = nested
        let! value = next
        return value
    }

The second let! is ambiguous.

Fix it with a type annotation:

let workflow : Flow<unit, string, int> =
    flow {
        let! next = nested
        let! (value: int) = next
        return value
    }

Error: The Flow Requires A Different Environment Type

This usually means you wrote a smaller workflow against one env type and are trying to run it inside a larger env.

Example:

type SmallEnv = { Prefix: string }
type BigEnv = { App: SmallEnv; RequestId: string }

let greet : Flow<SmallEnv, string, string> =
    flow {
        let! prefix = Flow.read _.Prefix
        return $"{prefix} world"
    }

If you want to run it in BigEnv, derive the smaller local env from the outer one:

let greetInBigEnv : Flow<BigEnv, string, string> =
    greet |> Flow.localEnv _.App

Error: Option Or ValueOption Does Not Match Your Error Type

Implicit option binding only works when the workflow error type is unit.

This fails:

let workflow : Flow<unit, string, int> =
    flow {
        let! value = Some 42
        return value
    }

Use an explicit adapter when you want a custom error:

let workflow : Flow<unit, string, int> =
    Some 42
    |> Flow.fromOption "missing value"

Error: ColdTask Does Not Match Task

Flow.fromTask and direct flow { let! ... } support ColdTask<'value> for delayed task work.

ColdTask<'value> means:

CancellationToken -> Task<'value>

Wrap the factory explicitly:

let load : ColdTask<int> =
    ColdTask(fun _ -> Task.FromResult 42)

If you already have a started Task<'value>, bind it directly in flow {} instead.

When Type Errors Usually Mean A Boundary Problem

If the compiler error mentions one of these shapes, check the boundary first:

• Result<...>
• Async<...>
• Async<Result<...>>
• Task<...>
• Task<Result<...>>
• Flow<...>

Most fixes are one of:

• add a type annotation to disambiguate let! overloads
• derive a smaller local environment with localEnv
• use Guard.Of or Guard.MapError to bridge existing error-bearing sources
• move back to plain Result until the real workflow boundary appears

Next

Read docs/GETTING_STARTED.md for the workflow overview, docs/TASK_ASYNC_INTEROP.md for the binding surface, and examples/FsFlow.MaintenanceExamples/Program.fs for runnable versions of these shapes.