trace-function-call: Easy lightweight tracing of function arguments and results for ad hoc debugging
Example:
You have a pure function that may be giving you incorrect results.
fib :: Int -> Int fib n | n < 2 = n | otherwise = fib (n-1) - fib (n-2)
>>>
fib 3
0
Insert a call to traceFunction
to aid with debugging.
fib, fib' :: Int -> Int fib = traceFunction "fib" fib' fib' n | n < 2 = n | otherwise = fib (n-1) - fib (n-2)
Calls to your pure function now provide its parameters and result as debugging information.
>>>
fib 3
fib 1 = 1 fib 0 = 0 fib 2 = 1 fib 1 = 1 fib 3 = 0 0
Hopefully this will help you home in on your bug.
Note that traceFunction
works with functions of more than one parameter...
traceElem :: Eq a => a -> [a] -> Bool traceElem = traceFunction "elem" elem
...and with "functions" of no parameters at all.
alpha = traceFunction "Fine-structure constant" $ e * e * c * mu0 / 2 / h
Parameters and results must implement the Show
typeclass. As a special case, parameters may instead be
functions, and are shown as an underscore (_
).
>>>
:set -XNoMonomorphismRestriction
>>>
let map' = traceFunction "map" map
>>>
map' (2 *) [1..3]
map _ [1,2,3] = [2,4,6] [2,4,6]
KNOWN BUG: The resultant function is strict, even when the input function is non-strict in some of its parameters. In particular,
if one of the parameters is
error "foo"
, the return value when the resultant function call is evaluated will beerror "foo"
; no trace message will be outputif one of the parameters doesn't terminate when evaluated, the resultant function call will not terminate when evaluated either; no trace message will be output
Downloads
- trace-function-call-0.1.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
Maintainer's Corner
For package maintainers and hackage trustees
Candidates
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Versions [RSS] | 0.1 |
---|---|
Dependencies | base (>=3.0.3.1 && <4.5) [details] |
License | BSD-3-Clause |
Copyright | © Dave Hinton 2011 |
Author | Dave Hinton |
Maintainer | beakerchu@googlemail.com |
Category | Debug |
Uploaded | by DaveHinton at 2011-12-20T07:23:25Z |
Distributions | |
Reverse Dependencies | 1 direct, 0 indirect [details] |
Downloads | 1164 total (3 in the last 30 days) |
Rating | (no votes yet) [estimated by Bayesian average] |
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Status | Docs uploaded by user Build status unknown [no reports yet] |