# trace-function-call: Easy lightweight tracing of function arguments and results for ad hoc debugging

[ bsd3, debug, library ] [ Propose Tags ]

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


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

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