I am writing some documentation with Haddock, and I need to put a multi-column table with some values in it. How can I do it with Haddock? I cannot find info about it. Embedding some html as alternative looks no possible too.

Haddock bundled with GHC 8.4 or newer (Haddock version >= 2.18.2) supports tables. As per the pull request where this was added, the syntax is based on RST Grid tables.

Sample use:

module Sample where

-- | A table:
--
-- +------------------------+------------+----------+----------+
-- | Header row, column 1   | Header 2   | Header 3 | Header 4 |
-- | (header rows optional) |            |          |          |
-- +========================+============+==========+==========+
-- | body row 1, column 1   | column 2   | column 3 | column 4 |
-- +------------------------+------------+----------+----------+
-- | body row 2             | Cells may span columns.          |
-- +------------------------+------------+---------------------+
-- | body row 3             | Cells may  | \[                  |
-- +------------------------+ span rows. | f(n) = \sum_{i=1}   |
-- | body row 4             |            | \]                  |
-- +------------------------+------------+---------------------+
sample :: ()
sample = ()

Turns into