Ignoring the return value of a function in F#

From time to time we need to call a function which has a return value which we don’t care about. E.g. a function can perform some action and then return true if it has succeeded but throw an exception otherwise. In that case it would be enough to call the function and ignore the return value assuming that the function succeeded in case no exception was thrown.

Here’s an extremely simple F# function which only returns true:

let booleanFunction someInput =

Here’s how we could call this function and keep its return value:

let booleanRes = booleanFunction "hello"

If we, instead, ignore the return value…:

booleanFunction "hello"

…then we get a warning and this line of code is marked with a blue line in Visual Studio:

This expression should have type ‘unit’, but has type ‘bool’. Use ‘ignore’ to discard the result of the expression, or ‘let’ to bind the result to a name.

As hinted at by the warning we can use something called “ignore”. The following shows what’s meant:

booleanFunction "hello" |> ignore

“ignore” is an F# operator in the Microsoft.FSharp.Core.Operators namespace and is used specifically to throw away the return value of a function.

View all F# related articles here.


About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

One Response to Ignoring the return value of a function in F#

  1. Philip S says:

    While you may be unable to avoid “true or throw” methods in third party libraries, please NEVER design your own methods in this manner. If your third party library dependency has “true or throw” methods, consider finding a better designed library.

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