Debug

Tracing strings

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.

>>> let x = 123; f = show
>>> trace ("calling f with x = " ++ show x) (f x)
"calling f with x = 123
123"

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.

traceId :: String -> String

Like trace but returns the message instead of a third value.

>>> traceId "hello"
"hello
hello"

Since: base-4.7.0.0

traceShow :: Show a => a -> b -> b

Like trace, but uses show on the argument to convert it to a String.

This makes it convenient for printing the values of interesting variables or expressions inside a function. For example here we print the value of the variables x and y:

>>> let f x y = traceShow (x,y) (x + y) in f (1+2) 5
(3,5)
8

traceShowId :: Show a => a -> a

Like traceShow but returns the shown value instead of a third value.

>>> traceShowId (1+2+3, "hello" ++ "world")
(6,"helloworld")
(6,"helloworld")

Since: base-4.7.0.0

traceStack :: String -> a -> a

like trace, but additionally prints a call stack if one is available.

In the current GHC implementation, the call stack is only available if the program was compiled with -prof; otherwise traceStack behaves exactly like trace. Entries in the call stack correspond to SCC annotations, so it is a good idea to use -fprof-auto or -fprof-auto-calls to add SCC annotations automatically.

Since: base-4.5.0.0

traceM :: Applicative f => String -> f ()

Like trace but returning unit in an arbitrary Applicative context. Allows for convenient use in do-notation.

Note that the application of traceM is not an action in the Applicative context, as traceIO is in the IO type. While the fresh bindings in the following example will force the traceM expressions to be reduced every time the do-block is executed, traceM "not crashed" would only be reduced once, and the message would only be printed once. If your monad is in MonadIO, liftIO . traceIO may be a better option.

>>> :{
do
    x <- Just 3
    traceM ("x: " ++ show x)
    y <- pure 12
    traceM ("y: " ++ show y)
    pure (x*2 + y)
:}
x: 3
y: 12
Just 18

Since: base-4.7.0.0

traceShowM :: (Show a, Applicative f) => a -> f ()

Like traceM, but uses show on the argument to convert it to a String.

>>> :{
do
    x <- Just 3
    traceShowM x
    y <- pure 12
    traceShowM y
    pure (x*2 + y)
:}
3
12
Just 18

Since: base-4.7.0.0

traceEvent :: String -> a -> a

The traceEvent function behaves like trace with the difference that the message is emitted to the eventlog, if eventlog profiling is available and enabled at runtime.

It is suitable for use in pure code. In an IO context use traceEventIO instead.

Note that when using GHC's SMP runtime, it is possible (but rare) to get duplicate events emitted if two CPUs simultaneously evaluate the same thunk that uses traceEvent.

Since: base-4.5.0.0

traceEventIO :: String -> IO ()

The traceEventIO function emits a message to the eventlog, if eventlog profiling is available and enabled at runtime.

Compared to traceEvent, traceEventIO sequences the event with respect to other IO actions.

Since: base-4.5.0.0

traceMarker :: String -> a -> a

The traceMarker function emits a marker to the eventlog, if eventlog profiling is available and enabled at runtime. The String is the name of the marker. The name is just used in the profiling tools to help you keep clear which marker is which.

This function is suitable for use in pure code. In an IO context use traceMarkerIO instead.

Note that when using GHC's SMP runtime, it is possible (but rare) to get duplicate events emitted if two CPUs simultaneously evaluate the same thunk that uses traceMarker.

Since: base-4.7.0.0

traceMarkerIO :: String -> IO ()

The traceMarkerIO function emits a marker to the eventlog, if eventlog profiling is available and enabled at runtime.

Compared to traceMarker, traceMarkerIO sequences the event with respect to other IO actions.

Since: base-4.7.0.0

Simulated call stack

data CallStack

CallStacks are a lightweight method of obtaining a partial call-stack at any point in the program.

A function can request its call-site with the HasCallStack constraint. For example, we can define

putStrLnWithCallStack :: HasCallStack => String -> IO ()

as a variant of putStrLn that will get its call-site and print it, along with the string given as argument. We can access the call-stack inside putStrLnWithCallStack with callStack.

putStrLnWithCallStack :: HasCallStack => String -> IO ()
putStrLnWithCallStack msg = do
  putStrLn msg
  putStrLn (prettyCallStack callStack)

Thus, if we call putStrLnWithCallStack we will get a formatted call-stack alongside our string.

>>> putStrLnWithCallStack "hello"
hello
CallStack (from HasCallStack):
  putStrLnWithCallStack, called at <interactive>:2:1 in interactive:Ghci1

GHC solves HasCallStack constraints in three steps:

1. If there is a CallStack in scope -- i.e. the enclosing function has a HasCallStack constraint -- GHC will append the new call-site to the existing CallStack.
2. If there is no CallStack in scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer a HasCallStack constraint for the enclosing definition (subject to the monomorphism restriction).
3. If there is no CallStack in scope and the enclosing definition has an explicit type signature, GHC will solve the HasCallStack constraint for the singleton CallStack containing just the current call-site.

CallStacks do not interact with the RTS and do not require compilation with -prof. On the other hand, as they are built up explicitly via the HasCallStack constraints, they will generally not contain as much information as the simulated call-stacks maintained by the RTS.

A CallStack is a [(String, SrcLoc)]. The String is the name of function that was called, the SrcLoc is the call-site. The list is ordered with the most recently called function at the head.

NOTE: The intrepid user may notice that HasCallStack is just an alias for an implicit parameter ?callStack :: CallStack. This is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Since: base-4.8.1.0

Instances

IsList CallStack

Be aware that 'fromList . toList = id' only for unfrozen CallStacks, since toList removes frozenness information.

Since: base-4.9.0.0

Defined in GHC.Exts

Associated Types

type Item CallStack :: *

Methods

fromList :: [Item CallStack] -> CallStack

fromListN :: Int -> [Item CallStack] -> CallStack

toList :: CallStack -> [Item CallStack]

Show CallStack

Since: base-4.9.0.0

Defined in GHC.Show

Methods

showsPrec :: Int -> CallStack -> ShowS

show :: CallStack -> String

showList :: [CallStack] -> ShowS

NFData CallStack

Since: deepseq-1.4.2.0

Defined in Control.DeepSeq

Methods

rnf :: CallStack -> ()

type Item CallStack

Defined in GHC.Exts

type Item CallStack = (String, SrcLoc)

currentCallStack :: IO [String]

Returns a [String] representing the current call stack. This can be useful for debugging.

The implementation uses the call-stack simulation maintained by the profiler, so it only works if the program was compiled with -prof and contains suitable SCC annotations (e.g. by using -fprof-auto). Otherwise, the list returned is likely to be empty or uninformative.

Since: base-4.5.0.0

whoCreated :: a -> IO [String]

Get the stack trace attached to an object.

Since: base-4.5.0.0

type HasCallStack = ?callStack :: CallStack

Request a CallStack.

NOTE: The implicit parameter ?callStack :: CallStack is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Since: base-4.9.0.0

callStack :: HasCallStack -> CallStack

Return the current CallStack.

Does *not* include the call-site of callStack.

Since: base-4.9.0.0

emptyCallStack :: CallStack

The empty CallStack.

Since: base-4.9.0.0

freezeCallStack :: CallStack -> CallStack

Freeze a call-stack, preventing any further call-sites from being appended.

pushCallStack callSite (freezeCallStack callStack) = freezeCallStack callStack

Since: base-4.9.0.0

fromCallSiteList :: [([Char], SrcLoc)] -> CallStack

Convert a list of call-sites to a CallStack.

Since: base-4.9.0.0

getCallStack :: CallStack -> [([Char], SrcLoc)]

Extract a list of call-sites from the CallStack.

The list is ordered by most recent call.

Since: base-4.8.1.0

popCallStack :: CallStack -> CallStack

Pop the most recent call-site off the CallStack.

This function, like pushCallStack, has no effect on a frozen CallStack.

Since: base-4.9.0.0

prettyCallStack :: CallStack -> String

Pretty print a CallStack.

Since: base-4.9.0.0

pushCallStack :: ([Char], SrcLoc) -> CallStack -> CallStack

Push a call-site onto the stack.

This function has no effect on a frozen CallStack.

Since: base-4.9.0.0

withFrozenCallStack :: HasCallStack => (HasCallStack -> a) -> a

Perform some computation without adding new entries to the CallStack.

Since: base-4.9.0.0

data SrcLoc

A single location in the source code.

Since: base-4.8.1.0

Constructors

SrcLoc

Fields

• srcLocPackage :: [Char]
• srcLocModule :: [Char]
• srcLocFile :: [Char]
• srcLocStartLine :: Int
• srcLocStartCol :: Int
• srcLocEndLine :: Int
• srcLocEndCol :: Int

Instances

Eq SrcLoc

Defined in GHC.Stack.Types

Methods

(==) :: SrcLoc -> SrcLoc -> Bool

(/=) :: SrcLoc -> SrcLoc -> Bool

Show SrcLoc

Defined in GHC.Show

Methods

showsPrec :: Int -> SrcLoc -> ShowS

show :: SrcLoc -> String

showList :: [SrcLoc] -> ShowS

NFData SrcLoc

Since: deepseq-1.4.2.0

Defined in Control.DeepSeq

Methods

rnf :: SrcLoc -> ()

prettySrcLoc :: SrcLoc -> String

Pretty print a SrcLoc.

Since: base-4.9.0.0

Execution stack (requires libdw)

data Location

Constructors

Location

Fields

• objectName :: String
• functionName :: String
• srcLoc :: Maybe (String, Int, Int)

getStackTrace :: IO (Maybe [Location])

showStackTrace :: IO (Maybe String)

Get a string representation of the current execution stack state.