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hspec aims to be a simple, extendable, and useful tool for Behavior Driven Development in Haskell. Step 1, write descriptions and examples of your desired behavior > module Myabs where > > specs :: IO Specs > specs = describe "myabs" [ > it "returns the original number when given a positive input" > (myabs 1 == 1), > > it "returns a positive number when given a negative input" > (myabs (-1) == 1), > > it "returns zero when given zero" > (myabs 0 == 0) > ] Step 2, write whatever you are describing > myabs n = undefined Step 3, watch your examples fail with red text by running from the .hs file itself > main = hspec specs myabs x returns the original number when given a positive input FAILED  x returns a positive number when given a negative input FAILED  x returns zero when given zero FAILED  1) myabs returns the original number when given a positive input FAILED Prelude.undefined 2) myabs returns a positive number when given a negative input FAILED Prelude.undefined 3) myabs returns zero when given zero FAILED Prelude.undefined Finished in 0.0002 seconds 3 examples, 3 failures Specs can also be run from the command line using the hspec program $ hspec myabs.hs Step 4, implement your desired behavior > myabs n = if n < 0 then negate n else n Step 5, watch your examples pass with green text when rerun myabs - returns the original number when given a positive input - returns a positive number when given a negative input - returns zero when given zero Finished in 0.0000 seconds 3 examples, 0 failures Here's the report of hspec's behavior: the "describe" function - takes a description of what the behavior is for - groups behaviors for what's being described a nested description - has it's own specs - may be displayed indented the "it" function - takes a description of a desired behavior - takes an example of that behavior - can use a Bool, HUnit Test, QuickCheck property, or "pending" as an example - will treat exceptions as failures the "hspec" function - displays a header for each thing being described - displays one row for each behavior - displays a '-' for successfull examples - displays an 'x' for failed examples - displays a detailed list of failed examples - displays a '-' for pending examples - displays a '#' and an additional message for pending examples - summarizes the time it takes to finish - summarizes the number of examples and failures - outputs failed examples in red, pending in yellow, and passing in green Bool as an example - is just an expression that evaluates to a Bool HUnit TestCase as an example - is specified with the HUnit "TestCase" data constructor - is the assumed example for IO() actions - will show the failed assertion text if available (e.g. assertBool) - will show the failed assertion expected and actual values if available (e.g. assertEqual) QuickCheck property as an example - is specified with the "property" function - will show what falsified it pending as an example - is specified with the "pending" function and an explanation - accepts a message to display in the report the "hHspecWithFormat" function - can use the "silent" formatter to show no output - can use the "progress" formatter to show '..F...FF.F' style output - can use the "specdoc" formatter to show all examples (default) - can use the "failed_examples" formatter to show only failed examples quantify (an internal function) - returns an amount and a word given an amount and word - returns a singular word given the number 1 - returns a plural word given a number greater than 1 - returns a plural word given the number 0 Finished in 0.0489 seconds 35 examples, 0 failures