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Why does this generate an error?
f#compiler
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let sumsecond data:(string*int) list = 
    data |> List.sum_by(fun (_,x) -> x)

error FS0001: The type '(string * int list)' does not support any operators named '+'.

However, sum_by is only supposed to take the sum of the results of the applying the function. In this case, it's clear that my anonymous function returns values of type int, which obviously has a (+) operator defined, so why is the compiler complaining about a (+) operator on string*int list?

.

You need parens:

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let sumsecond (data:(string*int) list) = 
    data |> List.sum_by (fun (_,x) -> x)

Try putting your original code in Visual Studio and hovering over 'data' - it has an unexpected type ('a list) which suggests that the type annotation you added was not doing what you expect. In your original example, you declared the return type of the function, not the type of the parameter.

By on 11/15/2008 10:53 PM ()

Funny, I tried every possible combination of parens EXCEPT that one :(

Thanks!

By on 11/15/2008 11:33 PM ()

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