Documentation

Response Header objects. You should never need to construct one directly. Instead, use addOptionalHeader.

addHeader adds a header to a response. Note that it changes the type of the value in the following ways:

1. A simple value is wrapped in "Headers '[hdr]":

>>> let example1 = addHeader 5 "hi" :: Headers '[Header "someheader" Int] String;
>>> getHeaders example1
[("someheader","5")]

1. A value that already has a header has its new header *prepended* to the existing list:

>>> let example1 = addHeader 5 "hi" :: Headers '[Header "someheader" Int] String;
>>> let example2 = addHeader True example1 :: Headers '[Header "1st" Bool, Header "someheader" Int] String
>>> getHeaders example2
[("1st","true"),("someheader","5")]

Note that while in your handlers type annotations are not required, since the type can be inferred from the API type, in other cases you may find yourself needing to add annotations.

Deliberately do not add a header to a value.

>>> let example1 = noHeader "hi" :: Headers '[Header "someheader" Int] String
>>> getHeaders example1
[]

Look up a specific ResponseHeader, without having to know what position it is in the HList.

>>> let example1 = addHeader 5 "hi" :: Headers '[Header "someheader" Int] String
>>> let example2 = addHeader True example1 :: Headers '[Header "1st" Bool, Header "someheader" Int] String
>>> lookupResponseHeader example2 :: ResponseHeader "someheader" Int
Header 5

>>> lookupResponseHeader example2 :: ResponseHeader "1st" Bool
Header True

Usage of this function relies on an explicit type annotation of the header to be looked up. This can be done with type annotations on the result, or with an explicit type application. In this example, the type of header value is determined by the type-inference, we only specify the name of the header:

>>> :set -XTypeApplications
>>> case lookupResponseHeader @"1st" example2 of { Header b -> b ; _ -> False }
True

Since: 0.15

Auxiliary class for GetHeaders (Headers hs a) instance