In the GHC 8.6.4, GHCi's `:doc` command does not work on ghc boot libraries, such as `GHC.Base`. Perhaps boot libraries of GHC 8.6.4 were compiled without the `-haddock` option. At least, it occurs in linux and windows binaries of GHC 8.6.4: * https://downloads.haskell.org/~ghc/8.6.4/ghc-8.6.4-x86_64-deb9-linux.tar.xz * https://downloads.haskell.org/~ghc/8.6.4/ghc-8.6.4-x86_64-unknown-mingw32.tar.xz The error messages are as follows: ``` ghci> :doc id ghc: Can't find any documentation for GHC.Base. This is probably because the module was compiled without '-haddock', but it's also possible that the module contains no documentation. Try re-compiling with '-haddock'. ``` ``` ghci> import Control.Monad.State ghci> :doc State ghc: Can't find any documentation for Control.Monad.Trans.State.Lazy. This is probably because the module was compiled without '-haddock', but it's also possible that the module contains no documentation. Try re-compiling with '-haddock'. ``` Previous version of GHCi (ghc 8.6.3) could perform `:doc` command as follows. ``` ghci> :doc id Identity function. > id x = x ``` ``` ghci> import Control.Monad.State ghci> :doc State A state monad parameterized by the type @s@ of the state to carry. The 'return' function leaves the state unchanged, while @>>=@ uses the final state of the first computation as the initial state of the second. ```