Debug.TraceUtils

This module exposes some useful tracing functions that should have been exported by Debug.Trace.

Feel free to copy&paste these functions into modules that need them, that may be easier to remove/clean up than adding a cabal dependency.

Documentation

traceId

Arguments

:: Show a

=> String

Prefix string to use before showing the value

-> a

-> a

Generate an identity function that has the side-effect of showing the value that passes through it.

Examples:

traceId "x,y = " (x, y)

traceIdVia

Arguments

:: Show b

=> (a -> b)

Function to preprocess the value before showing it

-> String

Prefix string to use before showing the result value

-> a

-> a

Generate an identity function that has the side-effect of tracing the value that passes through it by first processing it and then showing the result.

Examples:

traceIdVia (take 5) "First 5 sorted elements of: " result

fmap (traceIdVia objName "The object we got") . receiveObject

traceAround :: (Show i, Show o) => String -> (i -> o) -> i -> o

Convert a pure function to one that also has a side effect of tracing the value of the input and output values that pass through the function.

Examples:

traceAround "filterEntries" filterEntries entries

tracePutStrLn :: Monad m => String -> m ()

tracePrint :: (Show a, Monad m) => a -> m ()

trace :: String -> a -> a

The trace function outputs the trace message given as its first argument, before returning the second argument as its result.

For example, this returns the value of f x but first outputs the message.

 trace ("calling f with x = " ++ show x) (f x)

The trace function should only be used for debugging, or for monitoring execution. The function is not referentially transparent: its type indicates that it is a pure function but it has the side effect of outputting the trace message.