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Hoed is a lightweight tracer and algorithmic debugger that is practical to use for real-world programs.

Using Hoed

To locate a defect with Hoed you annotate suspected functions and compile as usual. Then you run your program, information about the annotated functions is collected. Finally you connect to a debugging session using a webbrowser.

Let us consider the following program, a defective implementation of a parity function with a test property.

isOdd :: Int -> Bool
isOdd n = isEven (plusOne n)

isEven :: Int -> Bool
isEven n = mod2 n == 0

plusOne :: Int -> Int
plusOne n = n + 1

mod2 :: Int -> Int
mod2 n = div n 2

prop_isOdd :: Int -> Bool
prop_isOdd x = isOdd (2*x+1)

Using the property-based test tool QuickCheck we find the counter example 1 for our property.

> quickCheck prop_isOdd
*** Failed! Falsifiable (after 1 test): 1

Hoed can help us determine which function is defective. We annotate the functions isOdd, isEven, plusOne and mod2 as follows:

import Debug.Hoed.Pure

isOdd :: Int -> Bool
isOdd = observe "isOdd" isOdd'
isOdd' n = isEven (plusOne n)

isEven :: Int -> Bool
isEven = observe "isEven" isEven'
isEven' n = mod2 n == 0

plusOne :: Int -> Int
plusOne = observe "plusOne" plusOne'
plusOne' n = n + 1

mod2 :: Int -> Int
mod2 = observe "mod2" mod2'
mod2' n = div n 2

prop_isOdd :: Int -> Bool
prop_isOdd x = isOdd (2*x+1)

Now we use the combinator testO to trace our program for the count-example of our property. After running the program a computation tree is constructed and displayed in a web browser.

> testO prop_isOdd 1
*** Failed! Falsifiable: 1
Listening on

You can freely browse this tree to get a better understanding of your program. If your program misbehaves, you can judge the computation statements in the tree as 'right' or 'wrong' according to your intention. When enough statements are judged the debugger tells you the location of the fault in your code.

Comparison with Other Tracers and Debuggers

Hat is probably the most advanced tracer tool for Haskell. It traces every reduction and provides many tools for viewing this trace. Hat requires a transformation of every module, even libraries we are not interested in. The transformation does not support as many language features as GHC and in practice many programs cannot be debugged with Hat. In contrast, Hoed is just a library and only functions we are interested in need to be annotated. Many programs that are difficult to debug with Hat can be debugged with Hoed.

Most Haskell implementations come with a trace primitive that can be used for printf style debugging. However, the primitive can force evaluation. Consider for example applying the function headDoubler> to [1..] in

headDoubler xs = trace ("headDoubler " ++ show xs) (2 * head (xs) : tail xs)

main = print (take 3 (headDoubler [1..]))

HOOD can be used like Debug.Trace for printf-style debugging while respecting evaluation order. Observing headDoubler in above example gives headDoubler (1:2:3:_) = 2:2:3:_. Hoed is based on HOOD, but gives a relation between computation statements and adds an algorithmic debugger.