Why can't I use let! (let bang) inside a try/with expression? - f#

Question

I'm making a call to an async function from inside a task {...} computation expression. I can normally use let! to call async functions and await their results. However, if I move the function call into a try/with expression, I get the following compiler error.
This construct may only be used within computation expressions
Since this code is still within the task{...} expression, I'm not sure what the compiler means exactly.
In the following contrived example the call to getDataMayThrowAsync from controller1 compiles, but in controller2 I get the above compile error.
module LetBangIssue =
open System
open System.Threading.Tasks
let getDataMayThrowAsync (id:int) = task {
do! Task.Delay(10)
let random = Random()
if random.Next(1, 1000) < 500 then
raise <| Exception "Something went wrong"
return "the data"
}
// Compiles, but doesn't handle error case...
let controller1() = task {
let! tryGetTheData = getDataMayThrowAsync 10
return tryGetTheData
}
// Attempt to handle error case, but can't compile...
let controller2 () = task {
let theData =
try
let! tryGetTheData = getDataMayThrowAsync 10
tryGetTheData
with
| ex ->
printfn $"Error: {ex.Message}"
"couldn't get the data"
return theData
}
How can I fix controller2 to solve the compile error?

Answer 1

It's not try/with that's the problem. It's the extra let. Try this:
let controller2 () = task {
let theData =
let! tryGetTheData = getDataMayThrowAsync 10
tryGetTheData
return theData
}
This fails with the same error, because in the body of theData you're using let! without a surrounding task { }.
As written, the extra let is not needed at all, since you're just returning it at the end. So this would work fine:
let controller2 () = task {
try
let! tryGetTheData = getDataMayThrowAsync 10
return tryGetTheData
with
| ex ->
printfn $"Error: {ex.Message}"
return "couldn't get the data"
}
But if you really need the extra let, you have to make it a let! (because it's bound to an async result), you have to wrap its body in task (otherwise you can't use let! inside it), and you have to use return to return the values from it:
let controller2 () = task {
let! theData = task {
try
let! tryGetTheData = getDataMayThrowAsync 10
return tryGetTheData
with
| ex ->
printfn $"Error: {ex.Message}"
return "couldn't get the data"
}
return theData
}