[Haskell-cafe] ANN: rematch, an library for composable assertions with human readable failure messages

Tom Crayford tcrayford at gmail.com
Tue Apr 16 11:17:48 CEST 2013

I kept on running into this thing where I was calling error in quickcheck
to get good error messages about the things I was comparing. In Java land,
this stuff is handled by Hamcrest: a library for composable assertions with
good error messages. This library is basically a port of hamcrest's core
api, but I've been very pleased with how it turned out.

I've been using this in tests for production code for a month or so now,
and I'm very pleased with it.

Running a matcher (in this example in an hunit test) looks like this:

expect [1] (is [1])

 The core API is very simple:

data Matcher a = Matcher {
    match :: a -> Bool
  -- ^ A function that returns True if the matcher should pass, False if it
should fail
  , description :: String
  -- ^ A description of the matcher (usually of its success conditions)
  , describeMismatch :: a -> String
  -- ^ A description to be shown if the match fails.

This means you can add/write your own matchers happily, which occasionally
means you can write *very* nice test code (here's an example of using a
custom matcher for checking the state of an "issue" in a hypothetical issue
tracking app):

expect latestIssue (hasState Resolved)

-- I removed the supporting code to make this assertion actually run,
-- this email is already pretty long.

There are numerous matchers (and functions for creating matchers) in the
rematch library, including some composition functions that provide good
failure messages.

There are some shims to hook rematch into the common haskell test
frameworks (specifically hunit and quickcheck).

The two libraries are up on hackage:

The code is all up on github:


I get rather frustrated when my tests give bad failure explanations, and
using rematch goes a long way to fix that.

Lastly, rematch is pretty isolated from test frameworks/etc, with a very
small and easy to understand surface api. Hopefully it'll help with the
thing I've seen in other languages (cough ruby cough) with every test
framework reinventing this idea, and not all frameworks having all the
matchers I want to use.

Let me know if you have any feedback/thoughts

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