trace-function-call

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 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