That feature requires the Haskel language server (which you can select during the Haskel installation with GHCup) and the VS code extension. If you have both of these installed and it still doesn't work, there might be a setting in VS code that you can change. However, it is not that bad if it does not work. You can look up the documentation of any function by searching for it on Hoogle hoogle.haskell.org.
when i hover the cursor to the function, it didnt lead to the documentation link. How to make the documentation link appear?
That feature requires the Haskel language server (which you can select during the Haskel installation with GHCup) and the VS code extension. If you have both of these installed and it still doesn't work, there might be a setting in VS code that you can change. However, it is not that bad if it does not work. You can look up the documentation of any function by searching for it on Hoogle hoogle.haskell.org.