Getting started with the streamly package

This guide will show you how to write a simple program that uses Streamly, using the cabal build manager for Haskell.

No prior knowledge of either Haskell or of cabal is needed. We do however assume that you are using a command-line shell on a POSIX operating system. If you are running Windows™ then you may need to run a command-line shell under msys and some of the commands below may also need to be changed in small ways to make them work.

Note: If you are new to Haskell you may find the Haskell Getting Started Guide to be useful.

If you know your way around Haskell, and have an up to date toolchain already installed, then you can jump straight to the section titled “Prepare Your Build Directory”.

Getting started with streamly using cabal

To get started, you will need a recent cabal binary to be installed on your system.

Installing cabal

A pre-packaged cabal binary may already be available for your operating system.

For example, on Debian GNU/Linux, you could install cabal using::

$ sudo apt install cabal-install

Installing ghc

You would also need to install the Glasgow Haskell Compiler ghc. In most operating systems, using the system’s package manager to install cabal would also bring in a version of ghc as a dependency.

Please verify that ghc is present:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2

If ghc isn’t present, please install it using your operating system’s package manager. For example, on Debian GNU/Linux, use:

$ sudo apt install ghc

Update cabal if necessary

Many operating systems ship an old version of cabal; in order to use streamly you should use cabal version 3.0 (or later).

You can check the installed version of cabal by using its --version flag:

$ which cabal
$ /usr/bin/cabal --version
cabal-install version
compiled using version of the Cabal library

In this example, the installed cabal binary is too old for our needs. It would need to be upgraded.

You can ask cabal to upgrade itself (please see

$ cabal install Cabal cabal-install
Resolving dependencies...
Downloading base16-bytestring-
... lots of build output ...
Building cabal-install-
Installed cabal-install-

cabal will usually place its compiled binaries inside the directory $HOME/.cabal/bin.

You can verify the version of your newly installed binary by using the --version flag again:

$ $HOME/.cabal/bin/cabal --version
cabal-install version
compiled using version of the Cabal library

If you have upgraded cabal, do be sure to specify $HOME/.cabal/bin early in your PATH shell variable, so that the new binary gets used instead of the older version on your system.

$ PATH=$HOME/.cabal/bin:$PATH

Once you have cabal version 3.0 (or later) successfully installed, please refresh its package list by using cabal update.

$ cabal update
Downloading the latest package list from

Prepare Your Build Directory

Next, create a build directory, so that you can use cabal’s isolated builds feature.

$ mkdir streamly-play
$ cd streamly-play

Run cabal init to create an initial set of project files:

$ cabal init --minimal --dependency base --dependency streamly

Generating LICENSE...
... other messages ...
Generating Main.hs...
Generating streamly-play.cabal...

Warning: no synopsis given. You should edit the .cabal file and add one.
You may want to edit the .cabal file and add a Description field.

This invocation will set up a build with two build dependencies, namely base and streamly. You can add additional dependencies later, by editing the build-depends section of the generated cabal file streamly-play.cabal. Please see the Cabal User Guide for more information on how to configure cabal.

This invocation also creates a skeletal Main.hs program which we will use later.

Try streamly in the GHCi REPL

You can now try out streamly using the GHCi interpreter’s read-eval-print-loop (REPL) facility.

To start up the GHCi REPL using the released streamly version on Hackage, please use:

$ cabal repl --build-depends streamly
... plenty of build messaages, the first time around ...
GHCi, version 8.8.3:  :? for help
[1 of 1] Compiling Main             ( Main.hs, interpreted )
Ok, one module loaded.

Once at the *Main> prompt, you can import streamly and use it directly:

*Main> import qualified Streamly.Prelude as Stream

*Main Stream> Stream.drain $ Stream.mapM print $ Stream.fromList [1..3]
*Main Stream>

For the curious, here is a high level overview of what these lines do:

  1. import qualified Streamly.Prelude as Stream imports the Streamly prelude into GHCi, and makes it available as module Stream.
  2. [1..3] generates the Haskell list [1, 2, 3].
  3. Stream.fromList transforms that list into a stream of integers.
  4. Stream.mapM print transforms the stream of integers into a stream of actions that would print those integers when executed.
  5. Stream.drain transforms that stream of actions into an IO action that main or GHCi’s REPL can execute.
Using a specific version of streamly in the REPL

You can also ask cabal to use a specific version of streamly by adding a version number constraint to the --build-depends flag:

$ cabal repl --build-depends streamly==0.8.0
[1 of 1] Compiling Main             ( Main.hs, interpreted )
Ok, modules loaded: Main.
(Advanced) Using the development version of streamly in the REPL

To use the development version of streamly, we need to configure cabal to fetch it from Github.

Create a cabal.project file in the current directory with the following content:

packages: .
  type: git
  tag: master

With this file present, cabal will fetch and build the current version of streamly from Github every time it is run. For example:

$ cabal repl
... fetches the 'master' branch of streamly from Github ...
... build messages ...
[1 of 1] Compiling Main     v        ( Main.hs, interpreted )
Ok, modules loaded: Main.

Using streamly in a standalone program

Let us now turn the single-line stanza above into a standalone program that uses streamly.

Edit Main.hs to contain the following:

module Main where

import qualified Streamly.Prelude as Stream

main :: IO ()
main = Stream.drain $ Stream.mapM print $ Stream.fromList [1..3]

Build and run this program using cabal run:

$ cabal run
... build messages ...

Which version of streamly should you use?

If you are new to Streamly, we recommend using latest stable release of streamly on Hackage.

If you need access to cutting edge features (and do not mind the occasional breakage), please use the development version of streamly on Github.

Next Steps

If you got this far successfully, congratulations!