GHCi contains a simple imperative-style debugger in which you can stop a running computation in order to examine the values of variables. The debugger is integrated into GHCi, and is turned on by default: no flags are required to enable the debugging facilities. There is one major restriction: breakpoints and single-stepping are only available in interpreted modules; compiled code is invisible to the debugger.
The debugger provides the following:
The ability to set a breakpoint on a function definition or expression in the program. When the function is called, or the expression evaluated, GHCi suspends execution and returns to the prompt, where you can inspect the values of local variables before continuing with the execution.
Execution can be single-stepped: the evaluator will suspend execution approximately after every reduction, allowing local variables to be inspected. This is equivalent to setting a breakpoint at every point in the program.
Execution can take place in tracing mode, in which the evaluator remembers each evaluation step as it happens, but doesn't suspend execution until an actual breakpoint is reached. When this happens, the history of evaluation steps can be inspected.
Exceptions (e.g. pattern matching failure and
error) can be treated as breakpoints, to help
locate the source of an exception in the program.
There is currently no support for obtaining a “stack trace”, but the tracing and history features provide a useful second-best, which will often be enough to establish the context of an error.
Let's use quicksort as a running example. Here's the code:
qsort  =  qsort (a:as) = qsort left ++ [a] ++ qsort right where (left,right) = (filter (<=a) as, filter (>a) as) main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
First, load the module into GHCi:
Prelude> :l qsort.hs [1 of 1] Compiling Main ( qsort.hs, interpreted ) Ok, modules loaded: Main. *Main>
Now, let's set a breakpoint on the right-hand-side of the second equation of qsort:
*Main> :break 2 Breakpoint 0 activated at qsort.hs:2:15-46 *Main>
:break 2 sets a breakpoint on line
2 of the most recently-loaded module, in this case
qsort.hs. Specifically, it picks the
leftmost complete subexpression on that line on which to set the
breakpoint, which in this case is the expression
(qsort left ++ [a] ++ qsort right).
Now, we run the program:
*Main> main Stopped at qsort.hs:2:15-46 _result :: [a] a :: a left :: [a] right :: [a] [qsort.hs:2:15-46] *Main>
Execution has stopped at the breakpoint. The prompt has changed to
indicate that we are currently stopped at a breakpoint, and the location:
[qsort.hs:2:15-46]. To further clarify the
location, we can use the
[qsort.hs:2:15-46] *Main> :list 1 qsort  =  2 qsort (a:as) = qsort left ++ [a] ++ qsort right 3 where (left,right) = (filter (<=a) as, filter (>a) as)
:list command lists the source code around
the current breakpoint. If your output device supports it, then GHCi
will highlight the active subexpression in bold.
GHCi has provided bindings for the free variables of the expression
on which the
breakpoint was placed (
right), and additionally a binding for the result of
the expression (
_result). These variables are just
like other variables that you might define in GHCi; you
can use them in expressions that you type at the prompt, you can ask
for their types with
:type, and so on. There is one
important difference though: these variables may only have partial
types. For example, if we try to display the value of
[qsort.hs:2:15-46] *Main> left <interactive>:1:0: Ambiguous type variable `a' in the constraint: `Show a' arising from a use of `print' at <interactive>:1:0-3 Cannot resolve unknown runtime types: a Use :print or :force to determine these types
This is because
qsort is a polymorphic function,
and because GHCi does not carry type information at runtime, it cannot
determine the runtime types of free variables that involve type
variables. Hence, when you ask to display
the prompt, GHCi can't figure out which instance of
Show to use, so it emits the type error above.
Fortunately, the debugger includes a generic printing command,
[qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show [qsort.hs:2:15-46] *Main> :print left left = (_t1::[a])
This isn't particularly enlightening. What happened is that
left is bound to an unevaluated computation (a
suspension, or thunk), and
Show instances when possible. This happens
only when the contents of the variable being inspected
are completely evaluated.
If we aren't concerned about preserving the evaluatedness of a
variable, we can use
:force instead of
behaves exactly like
[qsort.hs:2:15-46] *Main> :force left left = [4,0,3,1]
:force has inspected the runtime
left, it has reconstructed its type. We
can see the results of this type reconstruction:
[qsort.hs:2:15-46] *Main> :show bindings _result :: [Integer] a :: Integer left :: [Integer] right :: [Integer] _t1 :: [Integer]
Not only do we now know the type of
all the other partial types have also been resolved. So we can ask
for the value of
a, for example:
[qsort.hs:2:15-46] *Main> a 8
You might find it useful to use Haskell's
seq function to evaluate individual thunks rather
than evaluating the whole expression with
[qsort.hs:2:15-46] *Main> :print right right = (_t1::[Integer]) [qsort.hs:2:15-46] *Main> seq _t1 () () [qsort.hs:2:15-46] *Main> :print right right = 23 : (_t2::[Integer])
We evaluated only the
_t1 thunk, revealing the
head of the list, and the tail is another thunk now bound to
seq function is a
little inconvenient to use here, so you might want to use
:def to make a nicer interface (left as an exercise
for the reader!).
Finally, we can continue the current execution:
[qsort.hs:2:15-46] *Main> :continue Stopped at qsort.hs:2:15-46 _result :: [a] a :: a left :: [a] right :: [a] [qsort.hs:2:15-46] *Main>
The execution continued at the point it previously stopped, and has now stopped at the breakpoint for a second time.
Breakpoints can be set in various ways. Perhaps the easiest way to set a breakpoint is to name a top-level function:
identifier names any top-level
function in an interpreted module currently loaded into GHCi (qualified
names may be used). The breakpoint will be set on the body of the
function, when it is fully applied but before any pattern matching has
Breakpoints can also be set by line (and optionally column) number:
When a breakpoint is set on a particular line, GHCi sets the breakpoint on the leftmost subexpression that begins and ends on that line. If two complete subexpressions start at the same column, the longest one is picked. If there is no complete subexpression on the line, then the leftmost expression starting on the line is picked, and failing that the rightmost expression that partially or completely covers the line.
When a breakpoint is set on a particular line and column, GHCi
picks the smallest subexpression that encloses that location on which
to set the breakpoint. Note: GHC considers the TAB character to have a
width of 1, wherever it occurs; in other words it counts
characters, rather than columns. This matches what some editors do,
and doesn't match others. The best advice is to avoid tab
characters in your source code altogether (see
-fwarn-tabs in Section 5.7, “Warnings and sanity-checking”).
If the module is omitted, then the most recently-loaded module is used.
Not all subexpressions are potential breakpoint locations. Single variables are typically not considered to be breakpoint locations (unless the variable is the right-hand-side of a function definition, lambda, or case alternative). The rule of thumb is that all redexes are breakpoint locations, together with the bodies of functions, lambdas, case alternatives and binding statements. There is normally no breakpoint on a let expression, but there will always be a breakpoint on its body, because we are usually interested in inspecting the values of the variables bound by the let.
The list of breakpoints currently enabled can be displayed using
*Main> :show breaks  Main qsort.hs:1:11-12  Main qsort.hs:2:15-46
To delete a breakpoint, use the
command with the number given in the output from
*Main> :delete 0 *Main> :show breaks  Main qsort.hs:2:15-46
To delete all breakpoints at once, use
Single-stepping is a great way to visualise the execution of your
program, and it is also a useful tool for identifying the source of a
bug. GHCi offers two variants of stepping. Use
:step to enable all the
breakpoints in the program, and execute until the next breakpoint is
:steplocal to limit the set
of enabled breakpoints to those in the current top level function.
:stepmodule to single step only on
breakpoints contained in the current module.
*Main> :step main Stopped at qsort.hs:5:7-47 _result :: IO ()
begins the evaluation of
expr in single-stepping mode. If
expr is omitted, then it single-steps from
the current breakpoint.
:list command is particularly useful when
single-stepping, to see where you currently are:
[qsort.hs:5:7-47] *Main> :list 4 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18]) 6 [qsort.hs:5:7-47] *Main>
In fact, GHCi provides a way to run a command when a breakpoint is
hit, so we can make it automatically do
[qsort.hs:5:7-47] *Main> :set stop :list [qsort.hs:5:7-47] *Main> :step Stopped at qsort.hs:5:14-46 _result :: [Integer] 4 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18]) 6 [qsort.hs:5:14-46] *Main>
When GHCi is stopped at a breakpoint, and an expression entered at the prompt triggers a second breakpoint, the new breakpoint becomes the “current” one, and the old one is saved on a stack. An arbitrary number of breakpoint contexts can be built up in this way. For example:
[qsort.hs:2:15-46] *Main> :st qsort [1,3] Stopped at qsort.hs:(1,0)-(3,55) _result :: [a] ... [qsort.hs:(1,0)-(3,55)] *Main>
While stopped at the breakpoint on line 2 that we set earlier, we
started a new evaluation with
:step qsort [1,3].
This new evaluation stopped after one step (at the definition of
qsort). The prompt has changed, now prefixed with
..., to indicate that there are saved breakpoints
beyond the current one. To see the stack of contexts, use
... [qsort.hs:(1,0)-(3,55)] *Main> :show context --> main Stopped at qsort.hs:2:15-46 --> qsort [1,3] Stopped at qsort.hs:(1,0)-(3,55) ... [qsort.hs:(1,0)-(3,55)] *Main>
To abandon the current evaluation, use
... [qsort.hs:(1,0)-(3,55)] *Main> :abandon [qsort.hs:2:15-46] *Main> :abandon *Main>
When stopped at a breakpoint or single-step, GHCi binds the
_result to the value of the currently
active expression. The value of
presumably not available yet, because we stopped its evaluation, but it
can be forced: if the type is known and showable, then just entering
_result at the prompt will show it. However,
there's one caveat to doing this: evaluating
will be likely to trigger further breakpoints, starting with the
breakpoint we are currently stopped at (if we stopped at a real
breakpoint, rather than due to
:step). So it will
probably be necessary to issue a
immediately when evaluating
you can use
:force which ignores breakpoints.
A question that we often want to ask when debugging a program is “how did I get here?”. Traditional imperative debuggers usually provide some kind of stack-tracing feature that lets you see the stack of active function calls (sometimes called the “lexical call stack”), describing a path through the code to the current location. Unfortunately this is hard to provide in Haskell, because execution proceeds on a demand-driven basis, rather than a depth-first basis as in strict languages. The “stack“ in GHC's execution engine bears little resemblance to the lexical call stack. Ideally GHCi would maintain a separate lexical call stack in addition to the dynamic call stack, and in fact this is exactly what our profiling system does (Chapter 6, Profiling), and what some other Haskell debuggers do. For the time being, however, GHCi doesn't maintain a lexical call stack (there are some technical challenges to be overcome). Instead, we provide a way to backtrack from a breakpoint to previous evaluation steps: essentially this is like single-stepping backwards, and should in many cases provide enough information to answer the “how did I get here?” question.
To use tracing, evaluate an expression with the
:trace command. For example, if we set a breakpoint
on the base case of
*Main> :list qsort 1 qsort  =  2 qsort (a:as) = qsort left ++ [a] ++ qsort right 3 where (left,right) = (filter (<=a) as, filter (>a) as) 4 *Main> :b 1 Breakpoint 1 activated at qsort.hs:1:11-12 *Main>
and then run a small
*Main> :trace qsort [3,2,1] Stopped at qsort.hs:1:11-12 _result :: [a] [qsort.hs:1:11-12] *Main>
We can now inspect the history of evaluation steps:
[qsort.hs:1:11-12] *Main> :hist -1 : qsort.hs:3:24-38 -2 : qsort.hs:3:23-55 -3 : qsort.hs:(1,0)-(3,55) -4 : qsort.hs:2:15-24 -5 : qsort.hs:2:15-46 -6 : qsort.hs:3:24-38 -7 : qsort.hs:3:23-55 -8 : qsort.hs:(1,0)-(3,55) -9 : qsort.hs:2:15-24 -10 : qsort.hs:2:15-46 -11 : qsort.hs:3:24-38 -12 : qsort.hs:3:23-55 -13 : qsort.hs:(1,0)-(3,55) -14 : qsort.hs:2:15-24 -15 : qsort.hs:2:15-46 -16 : qsort.hs:(1,0)-(3,55) <end of history>
To examine one of the steps in the history, use
[qsort.hs:1:11-12] *Main> :back Logged breakpoint at qsort.hs:3:24-38 _result :: [a] as :: [a] a :: a [-1: qsort.hs:3:24-38] *Main>
Note that the local variables at each step in the history have been
preserved, and can be examined as usual. Also note that the prompt has
changed to indicate that we're currently examining the first step in
-1. The command
:forward can be used to traverse forward in the
:trace command can be used with or without
an expression. When used without an expression, tracing begins from
the current breakpoint, just like
The history is only available when
:trace; the reason for this is we found that
logging each breakpoint in the history cuts performance by a factor of
2 or more. GHCi remembers the last 50 steps in the history (perhaps in
the future we'll make this configurable).
Another common question that comes up when debugging is
“where did this exception come from?”. Exceptions such as
those raised by
have no context information attached to them. Finding which
particular call to
head in your program resulted in
the error can be a painstaking process, usually involving
Debug.Trace.trace, or compiling with
profiling and using
+RTS -xc (see Section 6.3, “Time and allocation profiling”).
The GHCi debugger offers a way to hopefully shed some light on
these errors quickly and without modifying or recompiling the source
code. One way would be to set a breakpoint on the location in the
source code that throws the exception, and then use
:history to establish
the context. However,
head is in a library and
we can't set a breakpoint on it directly. For this reason, GHCi
provides the flags
-fbreak-on-exception which causes
the evaluator to stop when an exception is thrown, and
-fbreak-on-error, which works similarly but stops only on
uncaught exceptions. When stopping at an exception, GHCi will act
just as it does when a breakpoint is hit, with the deviation that it
will not show you any source code location. Due to this, these
commands are only really useful in conjunction with
:trace, in order to log the steps leading up to the
exception. For example:
*Main> :set -fbreak-on-exception *Main> :trace qsort ("abc" ++ undefined) "Stopped at <exception thrown> _exception :: e [<exception thrown>] *Main> :hist -1 : qsort.hs:3:24-38 -2 : qsort.hs:3:23-55 -3 : qsort.hs:(1,0)-(3,55) -4 : qsort.hs:2:15-24 -5 : qsort.hs:2:15-46 -6 : qsort.hs:(1,0)-(3,55) <end of history> [<exception thrown>] *Main> :back Logged breakpoint at qsort.hs:3:24-38 _result :: [a] as :: [a] a :: a [-1: qsort.hs:3:24-38] *Main> :force as *** Exception: Prelude.undefined [-1: qsort.hs:3:24-38] *Main> :print as as = 'b' : 'c' : (_t1::[Char])
The exception itself is bound to a new variable,
Breaking on exceptions is particularly useful for finding out what your program was doing when it was in an infinite loop. Just hit Control-C, and examine the history to find out what was going on.
It is possible to use the debugger to examine function values. When we are at a breakpoint and a function is in scope, the debugger cannot show you the source code for it; however, it is possible to get some information by applying it to some arguments and observing the result.
The process is slightly complicated when the binding is polymorphic.
We show the process by means of an example.
To keep things simple, we will use the well known
import Prelude hiding (map) map :: (a->b) -> [a] -> [b] map f  =  map f (x:xs) = f x : map f xs
We set a breakpoint on
map, and call it.
*Main> :break 5 Breakpoint 0 activated at map.hs:5:15-28 *Main> map Just [1..5] Stopped at map.hs:(4,0)-(5,12) _result :: [b] x :: a f :: a -> b xs :: [a]
GHCi tells us that, among other bindings,
f is in scope.
However, its type is not fully known yet,
and thus it is not possible to apply it to any
arguments. Nevertheless, observe that the type of its first argument is the
same as the type of
x, and its result type is shared
As we demonstrated earlier (Section 3.5.1, “Breakpoints and inspecting variables”), the
debugger has some intelligence built-in to update the type of
f whenever the types of
_result are discovered. So what we do in this
x a bit, in order to recover both its type
and the argument part of
*Main> seq x () *Main> :print x x = 1
We can check now that as expected, the type of
has been reconstructed, and with it the
f has been too:
*Main> :t x x :: Integer *Main> :t f f :: Integer -> b
From here, we can apply f to any argument of type Integer and observe the results.
*Main> let b = f 10 *Main> :t b b :: b *Main> b <interactive>:1:0: Ambiguous type variable `b' in the constraint: `Show b' arising from a use of `print' at <interactive>:1:0 *Main> :p b b = (_t2::a) *Main> seq b () () *Main> :t b b :: a *Main> :p b b = Just 10 *Main> :t b b :: Maybe Integer *Main> :t f f :: Integer -> Maybe Integer *Main> f 20 Just 20 *Main> map f [1..5] [Just 1, Just 2, Just 3, Just 4, Just 5]
In the first application of
f, we had to do
some more type reconstruction
in order to recover the result type of
But after that, we are free to use
When stopped at a breakpoint, if you try to evaluate a variable that is already under evaluation, the second evaluation will hang. The reason is that GHC knows the variable is under evaluation, so the new evaluation just waits for the result before continuing, but of course this isn't going to happen because the first evaluation is stopped at a breakpoint. Control-C can interrupt the hung evaluation and return to the prompt.
The most common way this can happen is when you're evaluating a CAF (e.g. main), stop at a breakpoint, and ask for the value of the CAF at the prompt again.
Implicit parameters (see Section 8.7.2, “Implicit parameters”) are only available at the scope of a breakpoint if there is an explicit type signature.
 We originally provided bindings for all variables in scope, rather than just the free variables of the expression, but found that this affected performance considerably, hence the current restriction to just the free variables.