quickpull- Generate Main module with QuickCheck tests

Safe HaskellSafe-Inferred




Quickpull - a simple testing framework to reduce boilerplating

Quickpull is a testing framework that helps you test multiple QuickCheck properties with a minimum of boilerplating and redundancy. It consists of two parts: first, an executable file which reads Haskell modules and extracts properties from them; and second, this library, which runs the tests.

The executable, named quickpull by default, is a preprocessor. You have to run this executable to get the QuickCheck properties from your modules. It fetches single properties, which are values that have type Testable a, as well as trees of properties, which have type TestTree. For example, to get your properties if they are in modules in a directory named tests, run this from your project's root directory:

$ quickpull tests

The results will be printed to standard output. Inspect this output, then redirect the output to a module named Decrees.

quickpull does not know or parse Haskell. It simply reads the file and, following a few simple rules, copies the names of properties and trees, along with metadata about them. All single properties must be named beginning with prop_, and trees must be named beginning with proptree_. The name of the property or tree must be the first thing on the line, not preceded by spaces on the line (which is probably how you write modules anyway if you are using layout.)

One problem with this method is that code that is commented out using the block commenting (with braces) will still be read; at present a simple workaround is to comment out blocks of code using per-line commenting instead.

quickpull only gets tests from files ending in .hs, and the property name must be the very first thing on the line; therefore, literate Haskell files won't work.

You then need a program that actually runs your tests. To get started, you need just a short simple program, like this:

   module Main where

   import Decrees
   import Quickpull

   main = defaultMain decrees

When run, this program will run the tests, print the results to standard output, and exit with an appropriate error code.

If your needs are more complicated, study the Quickpull.Types and Quickpull.Runners modules. For example, you can:

  • change the arguments you pass to the QuickCheck driver
  • inspect each Decree separately to change the way the test is run for different Decrees
  • filter the list of Decree so that you run only some tests rather than all of them

There is also the Quickpull.Laws module, which helps you test common laws, such as the Monad laws and the Monoid laws.

Advantages of Quickpull:

  • reduces boilerplate and frees you from the tedious task of assembling many tests from one module, and of assembling tests from different modules into another module
  • compiles quickly; no heavyweight dependencies.
  • No Template Haskell.


  • Requires you to run a preprocessor to get the tests
  • No way to select at runtime which tests to run or the QuickCheck parameters (at least, not out of the box; you could add this.)
  • The preprocessor is dumb and inelegant.
  • No support for Smallcheck or HUnit.
  • Currently no way to test polymorphic properties; you will have to monomorphise these yourself.

For examples of usage, see the tests that are bundled with Quickpull in the tests/ directory. These tests use Quickpull to test some common laws, such as the Monad and Monoid laws, on types that come in the standard library. There is a similar test for the QuickCheck Gen monad in the quickcheck-tests/ directory; interestingly enough, Gen does not satisfy the monad laws. To compile and run the tests/ tests, run cabal with the --enable-tests option; it will produce a binary named quickpull-tests. For the quickcheck-tests/ directory, run Cabal with the -fbuild-test-gen option; it will produce a binary named quickpull-test-gen.

To see how to integrate Quickpull into a development workflow, see the Quickpull source on Github at


It has a generate script that runs the quickpull binary to generate the appropriate Decrees modules; after that, cabal can be used as usual to build the package, executables, and distribution tarball. That way, although your test suite will need Quickpull listed as a build dependency so the tests will build, your users will not need to do any preprocessing to run the tests.


Trees of Testable

data TestTree Source

A tree of tests. This allows you to group tests for convenient display; it also allows you to write computations that return entire groups of tests.


group Source


:: String

Group name; a string with no trailing newline.

-> [TestTree] 
-> TestTree 

Create a new Group of tests.

test Source


:: Testable a 
=> String

Test name; a string with no trailing newline.

-> a 
-> TestTree 

Create a new Test.

Test Article, Metadata

data ModDesc Source

Description of a module.




modPath :: String

Path to the module

modName :: [String]

Each part of the hierarchical name

data Meta Source

Metadata about a particular test or group.




modDesc :: ModDesc

Name of file providing this test or group.

linenum :: Int

Line number of the test or group.

qName :: String

The name of the test or group, such as prop_mytest or proptree_mytest.


data Article Source

A single property or tree to test.


forall a . Testable a => Single a 
Multi TestTree 

data Decree Source

Specifies a single item to test, along with metadata about that item.




meta :: Meta
item :: Article


Basic functions to run tests; for more complex needs, see Quickpull.Runners.

quickCheckTree :: TestTree -> IO () Source

Checks a TestTree and prints the result to standard output. Intended for use in a REPL; however, the QuickCheck docs in Test.QuickCheck contain warnings about how type defaulting in GHCi can cause types to silently default to (); all those warnings also apply to use of this function.

defaultMain :: [Decree] -> IO () Source

Tests each Decree and prints the results to standard output. Exits successfully if all tests succeeded; otherwise, exits unsuccessfully.

Not recommended for REPL use as this function will either kill your REPL when it's done or, in the case of recent GHC versions, issue an exception.

defaultMainWith :: Args -> [Decree] -> IO () Source

Like defaultMain but allows you to pass arguments to the QuickCheck driver.

Not recommended for REPL use as this function will either kill your REPL when it's done or, in the case of recent GHC versions, issue an exception.