diff --git a/doctests.hs b/doctests.hs
index 300898f6..72e3b8a1 100644
--- a/doctests.hs
+++ b/doctests.hs
@@ -2,13 +2,27 @@ module Main where
 
 import Build_doctests (flags, pkgs, module_sources)
 import Data.Foldable (traverse_)
-import System.Environment.Compat (unsetEnv)
+import System.Environment.Compat (getArgs, unsetEnv)
 import Test.DocTest (doctest)
 
+import qualified Data.List as List
+
 main :: IO ()
 main = do
+    cliArgs <- getArgs
+    let (cliArgs1, mods) =
+            if (List.null (filter (== "--modules") cliArgs))
+            then (cliArgs, module_sources)
+            else (List.delete "--modules" cliArgs, [])
+    let args =
+              [ "-outputdir=./_doctests/"
+              , "-fobject-code"
+              , "--fast"
+              ]
+            ++ cliArgs1
+            ++ flags
+            ++ pkgs
+            ++ mods
     traverse_ putStrLn args
     unsetEnv "GHC_ENVIRONMENT"
     doctest args
-  where
-    args = flags ++ ["-outputdir=./_doctests/", "-fobject-code"] ++ pkgs ++ module_sources
diff --git a/src/Streamly/Internal/Data/Stream/Ahead.hs b/src/Streamly/Internal/Data/Stream/Ahead.hs
index 84ae32f7..93a57808 100644
--- a/src/Streamly/Internal/Data/Stream/Ahead.hs
+++ b/src/Streamly/Internal/Data/Stream/Ahead.hs
@@ -619,7 +619,7 @@ infixr 6 `ahead`
 -- original streams, side effects will happen in the order in which the streams
 -- are evaluated:
 --
--- >>> import Streamly.Prelude (ahead)
+-- >>> import Streamly.Prelude (ahead, SerialT)
 -- >>> stream1 = Stream.fromEffect (delay 4) :: SerialT IO Int
 -- >>> stream2 = Stream.fromEffect (delay 2) :: SerialT IO Int
 -- >>> Stream.toList $ stream1 `ahead` stream2 :: IO [Int]
diff --git a/src/Streamly/Internal/Data/Stream/Serial.hs b/src/Streamly/Internal/Data/Stream/Serial.hs
index 2d3283fe..ed10bdf2 100644
--- a/src/Streamly/Internal/Data/Stream/Serial.hs
+++ b/src/Streamly/Internal/Data/Stream/Serial.hs
@@ -281,6 +281,7 @@ TRAVERSABLE_INSTANCE(SerialT)
 -- element @1@ in the first stream with all the nested iterations of element
 -- @2@:
 --
+-- >>> import Streamly.Prelude (wSerial)
 -- >>> Stream.toList $ Stream.fromList [(1,3),(1,4)] `wSerial` Stream.fromList [(2,3),(2,4)]
 -- [(1,3),(2,3),(1,4),(2,4)]
 --
diff --git a/test/README.md b/test/README.md
index d7ee4600..af59bd52 100644
--- a/test/README.md
+++ b/test/README.md
@@ -1,4 +1,6 @@
-## Using bin/test.sh
+# Streamly Tests
+
+## Running tests using bin/test.sh
 
 `bin/test.sh` (path relative to the top level repo dir) can run selected
 test groups, generate coverage report, pass GHC RTS options when
@@ -15,7 +17,9 @@ To view coverage report, open `./hpc_index.html` in browser.
 
 See `bin/test.sh --help` for more info.
 
-## Build a single test
+## Building and Running Tests using cabal
+
+### Build a single test
 
 ```
 $ cabal build test:Prelude.Serial
@@ -33,7 +37,7 @@ Build with optimizations:
 $ cabal build --flag opt ...
 ```
 
-## Build all tests
+### Build all tests
 
 ```
 $ cabal build --enable-tests all
@@ -51,7 +55,7 @@ Or this, note this command does not work as expected when in the "test" dir:
 $ cabal build --enable-tests streamly-tests
 ```
 
-## Build and run
+### Build and run
 
 Running all test suites, use any of the following:
 
@@ -71,7 +75,7 @@ Or this, note this command does not work as expected when in the "test" dir:
 $ cabal test streamly-tests
 ```
 
-## Build and Run a single test suite
+### Build and Run a single test suite
 
 To run `Prelude.Serial` test-suite:
 
@@ -88,6 +92,63 @@ $ cd test; cabal run Prelude.Serial
 Note you could use `cabal test Prelude.Serial` but that unfortunately builds
 all the test suites before running `Prelude.Serial`.
 
+## Writing doctests
+
+* The test named `doctest` runs all the code snippets in a source module
+  that are written using the `>>>` markup in haddock. See `doctest.hs`.
+* Make sure you do not enclose your snippets in the `@ .. @` markup otherwise
+  they will show up verbatim in the docs and not as ghci styled snippets.
+* We use `--fast` mode of doctest, which means snippets are run as if you are
+  typing those examples from top to bottom in that order in GHCi. Previous
+  snippet's state is available to the next one.
+* A haddock section named `$setup` contains a snippet that is always run before
+  any other. When `--fast` mode is not used it is run before every snippet.
+
+An example setup section:
+
+```
+-- $setup
+-- >>> :m
+-- >>> import Control.Monad.IO.Class (MonadIO(..))
+-- >>> import Data.Function ((&))
+```
+
+Some tests that may take long can be written as follows.  Just assigning
+the code to a function makes it compile but not run.
+
+```
+>>> :{
+main = do
+ Stream.drain $ Stream.fromAsync $ foldMap delay [1..10]
+ Stream.drain $ Stream.concatFoldableWith Stream.async (map delay [1..10])
+ Stream.drain $ Stream.concatMapFoldableWith Stream.async delay [1..10]
+ Stream.drain $ Stream.concatForFoldableWith Stream.async [1..10] delay
+:}
+```
+
+## Running doctests
+
+Run tests for all modules:
+
+```
+$ cabal run doctests --flag doctests
+```
+
+Use verbose mode to debug:
+
+```
+$ cabal run doctests --flag doctests -- --verbose
+```
+
+Run tests only for selected modules:
+
+```
+$ cabal run doctests --flag doctests -- --modules Streamly.Prelude
+```
+
+If it fails with a message that a particular modules is not loaded specify that
+module as well on the command line.
+
 ## Naming of test modules
 
 Tests are organized by source modules. For example, for the source