GHC.Stack

base-4.12.0.0: Basic libraries

Copyright | (c) The University of Glasgow 2011
License | see libraries/base/LICENSE
Maintainer | cvs-ghc@haskell.org
Stability | internal
Portability | non-portable (GHC Extensions)
Safe Haskell | Trustworthy
Language | Haskell2010

GHC.Stack

Contents

- Profiling call stacks
- HasCallStack call stacks
- Source locations
- Internals

Description

Access to GHC's call-stack simulation

Since: base-4.5.0.0

Synopsis

- errorWithStackTrace :: String -> a
- currentCallStack :: IO [String]
- whoCreated :: a -> IO [String]
- data CallStack
- type HasCallStack = ?callStack :: CallStack
- callStack :: HasCallStack => CallStack
- emptyCallStack :: CallStack
- freezeCallStack :: CallStack -> CallStack
- fromCallSiteList :: [([Char], SrcLoc)] -> CallStack
- getCallStack :: CallStack -> [([Char], SrcLoc)]
- popCallStack :: CallStack -> CallStack
- prettyCallStack :: CallStack -> String
- pushCallStack :: ([Char], SrcLoc) -> CallStack -> CallStack
- withFrozenCallStack :: HasCallStack => (HasCallStack => a) -> a
- data SrcLoc = SrcLoc {
  - srcLocPackage :: [Char]
  - srcLocModule :: [Char]
  - srcLocFile :: [Char]
  - srcLocStartLine :: Int
  - srcLocStartCol :: Int
  - srcLocEndLine :: Int
  - srcLocEndCol :: Int
  }
- prettySrcLoc :: SrcLoc -> String
- data CostCentreStack
- data CostCentre
- getCurrentCCS :: dummy -> IO (Ptr CostCentreStack)
- getCCSOf :: a -> IO (Ptr CostCentreStack)
- clearCCS :: IO a -> IO a
- ccsCC :: Ptr CostCentreStack -> IO (Ptr CostCentre)
- ccsParent :: Ptr CostCentreStack -> IO (Ptr CostCentreStack)
- ccLabel :: Ptr CostCentre -> IO CString
- ccModule :: Ptr CostCentre -> IO CString
- ccSrcSpan :: Ptr CostCentre -> IO CString
- ccsToStrings :: Ptr CostCentreStack -> IO [String]
- renderStack :: [String] -> String

# Documentation

errorWithStackTrace :: String -> a #

Deprecated: `error` appends the call stack now

Like the function `error`, but appends a stack trace to the error message if one is available.

Since: base-4.7.0.0

# Profiling call stacks

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

# HasCallStack call stacks

data CallStack #

`CallStack`s 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.

`CallStack`s 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 `CallStack`s, since `toList` removes frozenness information. Since: base-4.9.0.0
Instance details Defined in GHC.Exts Associated Types type Item CallStack :: Type # Methods fromList :: [Item CallStack] -> CallStack # fromListN :: Int -> [Item CallStack] -> CallStack # toList :: CallStack -> [Item CallStack] #
Show CallStack # | Since: base-4.9.0.0
Instance details Defined in GHC.Show Methods showsPrec :: Int -> CallStack -> ShowS # show :: CallStack -> String # showList :: [CallStack] -> ShowS #
type Item CallStack # |
Instance details Defined in GHC.Exts type Item CallStack = (String, SrcLoc)

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

# Source locations

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 # | Since: base-4.9.0.0
Instance details Defined in GHC.Stack.Types Methods (==) :: SrcLoc -> SrcLoc -> Bool Source # (/=) :: SrcLoc -> SrcLoc -> Bool Source #
Show SrcLoc # | Since: base-4.9.0.0
Instance details Defined in GHC.Show Methods showsPrec :: Int -> SrcLoc -> ShowS # show :: SrcLoc -> String # showList :: [SrcLoc] -> ShowS #

prettySrcLoc :: SrcLoc -> String #

Pretty print a `SrcLoc`.

Since: base-4.9.0.0

# Internals

data CostCentreStack #

A cost-centre stack from GHC's cost-center profiler.

data CostCentre #

A cost-centre from GHC's cost-center profiler.

getCurrentCCS :: dummy -> IO (Ptr CostCentreStack) #

Returns the current `CostCentreStack` (value is `nullPtr` if the current program was not compiled with profiling support). Takes a dummy argument which can be used to avoid the call to `getCurrentCCS` being floated out by the simplifier, which would result in an uninformative stack (CAF).

getCCSOf :: a -> IO (Ptr CostCentreStack) #

Get the `CostCentreStack` associated with the given value.

clearCCS :: IO a -> IO a #

Run a computation with an empty cost-center stack. For example, this is used by the interpreter to run an interpreted computation without the call stack showing that it was invoked from GHC.

ccsCC :: Ptr CostCentreStack -> IO (Ptr CostCentre) #

Get the `CostCentre` at the head of a `CostCentreStack`.

ccsParent :: Ptr CostCentreStack -> IO (Ptr CostCentreStack) #

Get the tail of a `CostCentreStack`.

ccLabel :: Ptr CostCentre -> IO CString #

Get the label of a `CostCentre`.

ccModule :: Ptr CostCentre -> IO CString #

Get the module of a `CostCentre`.

ccSrcSpan :: Ptr CostCentre -> IO CString #

Get the source span of a `CostCentre`.

ccsToStrings :: Ptr CostCentreStack -> IO [String] #

Format a `CostCentreStack` as a list of lines.

renderStack :: [String] -> String #