I'm trying to create a custom workflow but to my astonishment it never invokes the Let member on my workflow builder. Even after carefully matching its signature with the definitions in the Expert F# book.

I've tried the sample in Expert F# with the Attempt monad (Chapter 9, Example 11, Listing 9-6) and stepped through but where it says 'let sum = n1 + n2' inside the Attempt workflow it does not invoke the Let member. All the others (Return, Bind and Delay) are called. The book says that

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attempt {
    let! n1 = failIfBig inp1
    let! n2 = failIfBig inp2
    let sum = n1 + n2
    return sum
}

is de-sugared into:

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attempt.Bind(failIfBig inp1, (fun n1 ->
    attempt.Bind(failIfBig inp2, (fun n2 ->
        <b>attempt.Let</b>(n1 + n2, (fun sum ->
            attempt.Return(sum)))))))

So it should invoke Let but it doesn't. It does invoke Delay however while it doesn't show in the de-sugared code.

Also the Expert F# book says that Let and Delay are required methods but it also says that these can be defined in terms of Bind and Return. Not defining Delay shows different behavior but not defining Let makes no difference.

Were there any significant changes between the F# version of the book and the one I'm using (1.9.6.2)? Or am I missing something obvious or less obvious here?