The Hugs 98 User's Guide
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# 7.4. Hugs debugging primitives

Hugs contains support for debugging by observations inspired by the Andy Gill's Hood library:

1. Andy Gill, Debugging Haskell by Observing Intermediate Data Structures, in Draft Proceedings of the 2000 Haskell Workshop.

2. The Haskell Object Observation Debugger http://www.haskell.org/hood/.

Hood is a portable Haskell library that implements the combinator

    observe :: Observable a => String -> a -> a

The partial application

    observe tag

behaves exactly like the identity function, but also records the value of data to which it is applied. Any observations made are reported at the end of the computation. The `tag` argument is used to label the observed value when it is reported. Non-strict semantics is preserved — `observe` does not evaluate its second argument.

HugsHood uses the same observation model but differs in a number of ways.

- It is much faster. This is because HugsHood is implemented within the Hugs evaluator and uses primitive builtin functions. Performance depends upon the volume of observations. More frequent observations incur a higher overhead. As a simple comparison, a test program which executed 1 million reductions and made 250 observations incurred a 625 percent overhead when observations were made with the Hood library but just 10 percent when using HugsHood.

  Caveat: When not using observations, the modifications to the evaluator to support HugsHood imposes an overhead of about 6 percent.

- It is possible to easily observe arbitrary data structures. HugsHood implements the primitive

      observe :: String -> a -> a

  which is unconstrained by the need to build instances of the `Observable` class for each user defined data type whose values are being observed. HugsHood uses an internal primitive function to display observed values. This may be considered both an advantage and a disadvantage: one does not need to define how to observe values, but one cannot define special user views of data.

- No modification to the program (apart from instrumentation with `observe`) is required. The Hood library must be invoked using a special IO monadic combinator to ensure that observations are collected and displayed.

- There are a number of minor differences in the display format which are a consequence of the Hugs implementation. These are described below.

## 7.4.1. Using HugsHood

Modules that use HugsHood combinators must import the module `Hugs.Observe`. Its only role is to provide the necessary primitive definitions, namely:

    primitive observe :: String -> a -> a
    primitive bkpt    :: String -> a -> a
    primitive setBkpt :: String -> Bool -> IO ()

### 7.4.1.1. Breakpoints

HugsHood implements breakpoints. A program can be instrumented with the `bkpt` function. The partial application

    bkpt bkpt_name

behaves exactly like the identity function, except that before it returns its argument it checks if `bkpt_name` is enabled, and if it is the user is presented with the opportunity to view observed data. A small set of commands is available when Hugs halts due to a breakpoint:

p [`tag_name`]

  Print observations made since the computation began. If an observation tag is suppled then only the associated observations will be displayed. Otherwise all observations will be displayed.

c [`n`]

  Continue with program evaluation. With no arguments, evaluation will continue until another active breakpoint is encountered. The optional numeric argument will skip `n` active breakpoints before stopping.

s `bkpt_name`

  Set a breakpoint.

r [`bkpt_name`]

  Reset a named breakpoint or, if no breakpoint name is supplied, reset all breakpoints.

A breakpoint is by default disabled. It can be enabled by using the s command in the debug breakpoint dialogue, or by using the `setBkpt` combinator. Clearly at least one breakpoint must be enabled using `setBkpt` before a breakpoint dialogue can be triggered.

### 7.4.1.2. Breakpoint Example

Here is a very simple program using the three combinators.

    import Hugs.Observe

    prog n = do { setBkpt "fib" True; putStr $ show (observe "fun" f n) }
    f 0 = 1
    f n = n * (bkpt "fib" $ observe "fun" f (n-1))

The following sample session shows how the p and c commands can be used.

    Main> prog 4
    Break @ fib> p

    >>>>>>> Observations <<<<<<

    fun
      { \ 4  -> _
      }

    Break @ fib> c
    Break @ fib> p

    >>>>>>> Observations <<<<<<

    fun
      { \ 4  -> _
      , \ 3  -> _
      }

    Break @ fib> c 2
    Break @ fib> p

    >>>>>>> Observations <<<<<<

    fun
      { \ 4  -> _
      , \ 3  -> _
      , \ 2  -> _
      , \ 1  -> _
      }

    Break @ fib> c
    24
    (98 reductions, 299 cells)

    >>>>>>> Observations <<<<<<

    fun
      { \ 4  -> 24
      , \ 3  -> 6
      , \ 2  -> 2
      , \ 1  -> 1
      , \ 0  -> 1
      }

    10 observations recorded

## 7.4.2. Differences from Hood

HugsHood uses a similar style of display to Hood, though there are differences. One trivial difference is that Hood reports tags with a leading "`--`" while HugsHood does not.

Consider now more significant differences.

### 7.4.2.1. Observing character strings

HugsHood (and Hood) reports lists using the cons operator.

    Observe> observe "list" [1..3]
    [1,2,3]

    >>>>>>> Observations <<<<<<

    list
      (1 : 2 : 3 : [])

This is too verbose for lists of characters, so HugsHood reports strings in the usual format:

    Observe> observe "string" ['a'..'d']
    "abcd"

    >>>>>>> Observations <<<<<<

    string
      "abcd"

If only the initial part of the string is evaluated, a trailing "`...`" is reported.

    Observe> take 2  $ observe "string" ['a'..'d']
    "ab"

    >>>>>>> Observations <<<<<<

    string
      "ab..."

This is clearly ambiguous, because evaluating the expression

    observe "string" "ab..."

will give the same result, but in practice the ambiguity should be easy to resolve.

### 7.4.2.2. Unevaluated expressions

The "`_`" symbol is used to indicate an unevaluated expression. In Hood all unevaluated expressions will be displayed using "`_`". In HugsHood, "`_`" denotes an unevaluated expression, but not all unevaluated expressions are denoted by "`_`".

For example the expression `fst $ observe "pair" (1,2)` yields

    -- pair
      (1, _)

in both Hugs and HugsHood. However, `fst $ observe "pair" ('a','b')` yields

    pair
      ('a','b')

in HugsHood, and `('a', _)` in Hood. This is because HugsHood (unlike Hood) does not actually record evaluation steps. It merely maintains an internal pointer to that part of the heap representing the tagged expression. If the expression in not in weak head normal form, then it obviously has not been evaluated and so it is reported as just "`_`"; otherwise it displayed. Integer constants like `1` and `2` are not in WHNF, as they must be coerced to the correct type when evaluated. Characters though are in WHNF so it is not possible to discern whether a character was evaluated.

Another consequence of the HugsHood implementation by pointers rather than Hood's implementation by tracing evaluation is that the strictness behaviour of a function can be masked. Consider the example:

    lazy pair = let x = observe "fst" fst pair
                    y = snd pair
                in  (y,x)

For the expression `lazy (1,2)` Hood reports

    -- fst
      { \ (1, _)  -> 1
      }

while HugsHood reports

    fst
      { \ (1,2)  -> 1
      }

HugsHood should not be used to deduce the strictness behaviour of a function, or it should be done only with caution.

### 7.4.2.3. Interaction with the root optimisation

The Hugs compiler uses an optimisation when generating code that builds expressions on the heap. If a function definition has the form

    f arg1 .. argN = ..... f arg1 .. argM .....

where 1 ≤ `M` ≤ `N`, then the expression graph for `f arg1 .. argM` is copied rather than rebuilt from individual application nodes. This interacts with the observation algorithm so that observing functions of the above form gives unexpected results.

For instance consider the expression

    observe "fold" foldl (+) 0 [1..3]

When the root optimisation is applied to the compilation of `foldl`, we see

    fold
      { \ primPlusInteger 6 []  -> 6
      , \ { \ 3 3  -> 6
          } 3 (3 : [])  -> 6
      , \ { \ 1 2  -> 3
          } 1 (2 : 3 : [])  -> 6
      , \ { \ 0 1  -> 1
          } 0 (1 : 2 : 3 : [])  -> 6

instead of the expected

    fold
      { \ { \ 0 1  -> 1
          , \ 1 2  -> 3
          , \ 3 3  -> 6
          } 0 (1 : 2 : 3 : [])  -> 6
      }

The first form reports the arguments at each application of `foldl`, while the second reports the arguments for just the initial application (the one marked by `observe`).

The root optimisation can be disabled using the `-R` option. This can be done from the command line or by using :s `-R` at the Hugs prompt. If you want to compile the prelude definitions without the root optimisation you must invoke Hugs with the `-R` option.

Testing of execution time with and without the root optimisation for a selection of 23 benchmarks from the nofib suite has been carried out. All but 5 tests resulted in an execution time penalty of less than 3% when running without root optimisation (some even showed a very minor speedup).

### 7.4.2.4. Known problems

Hugs can produce infinite (cyclic) dictionaries when implementing overloading. The observation reporting mechanism does not detect these at present, which leads to a non-terminating report. We plan to address this in a future release.

## 7.4.3. Reporting HugsHood bugs

Please report bugs to Richard Watson, `<rwatson@usq.edu.au>`

In particular, if the message

    Warning: observation sanity counter > 0

appears, and your program has not terminated abnormally, please report the error situation.

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