trace-function-call: Easy lightweight tracing of function arguments and results for ad hoc debugging
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)
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 3fib 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.
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 (
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 be
error "foo"; no trace message will be output
if 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
|Dependencies||base (>=18.104.22.168 && <4.5) [details]|
|Copyright||© Dave Hinton 2011|
|Uploaded||by DaveHinton at Tue Dec 20 07:23:25 UTC 2011|
|Downloads||523 total (8 in the last 30 days)|
|Rating||(no votes yet) [estimated by rule of succession]|
|Status||Docs uploaded by user
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