Overwrite response header in Yesod

How can I overwrite a previously set response header in Yesod? When I use the function addHeader

, it adds another header with the same name instead of overwriting it. For example, if inside a handler function, I do this:

addHeader "foo" "bar"
addHeader "foo" "baz"

      

        
        
        
      

    

I get

foo: bar
foo: baz

      

        
        
        
      

    

but I only want to get

foo: baz

      

        
        
        
      

    

In my case, the header is an authentication token, and it works in such a way that if it is present in the request, I send it back in the response. I have implemented this as middleware. However, if it is a login request, a new token is generated and should be sent instead of the value from the request header.