GHC/Err.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude, MagicHash, ImplicitParams #-}
    {-# LANGUAGE RankNTypes, TypeInType #-}
    {-# OPTIONS_HADDOCK hide #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Err
    -- Copyright   :  (c) The University of Glasgow, 1994-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC extensions)
    --
    -- The "GHC.Err" module defines the code for the wired-in error functions,
    -- which have a special type in the compiler (with \"open tyvars\").
    --
    -- We cannot define these functions in a module where they might be used
    -- (e.g., "GHC.Base"), because the magical wired-in type will get confused
    -- with what the typechecker figures out.
    --
    -----------------------------------------------------------------------------

    module GHC.Err( absentErr, error, errorWithoutStackTrace, undefined ) where
    import GHC.CString ()
    import GHC.Types (Char, RuntimeRep)
    import GHC.Stack.Types
    import GHC.Prim
    import GHC.Integer ()   -- Make sure Integer is compiled first
                            -- because GHC depends on it in a wired-in way
                            -- so the build system doesn't see the dependency
    import {-# SOURCE #-} GHC.Exception( errorCallWithCallStackException )

    -- | 'error' stops execution and displays an error message.
    error :: forall (r :: RuntimeRep). forall (a :: TYPE r).
             HasCallStack => [Char] -> a
    error s = raise# (errorCallWithCallStackException s ?callStack)
              -- Bleh, we should be using 'GHC.Stack.callStack' instead of
              -- '?callStack' here, but 'GHC.Stack.callStack' depends on
              -- 'GHC.Stack.popCallStack', which is partial and depends on
              -- 'error'.. Do as I say, not as I do.

    -- | A variant of 'error' that does not produce a stack trace.
    --
    -- @since 4.9.0.0
    errorWithoutStackTrace :: forall (r :: RuntimeRep). forall (a :: TYPE r).
                              [Char] -> a
    errorWithoutStackTrace s =
      -- we don't have withFrozenCallStack yet, so we just inline the definition
      let ?callStack = freezeCallStack emptyCallStack
      in error s

    -- Note [Errors in base]
    -- ~~~~~~~~~~~~~~~~~~~~~
    -- As of base-4.9.0.0, `error` produces a stack trace alongside the
    -- error message using the HasCallStack machinery. This provides
    -- a partial stack trace, containing the call-site of each function
    -- with a HasCallStack constraint.
    --
    -- In base, however, the only functions that have such constraints are
    -- error and undefined, so the stack traces from partial functions in
    -- base will never contain a call-site in user code. Instead we'll
    -- usually just get the actual call to error. Base functions already
    -- have a good habit of providing detailed error messages, including the
    -- name of the offending partial function, so the partial stack-trace
    -- does not provide any extra information, just noise. Thus, we export
    -- the callstack-aware error, but within base we use the
    -- errorWithoutStackTrace variant for more hygienic error messages.

    -- | A special case of 'error'.
    -- It is expected that compilers will recognize this and insert error
    -- messages which are more appropriate to the context in which 'undefined'
    -- appears.
    undefined :: forall (r :: RuntimeRep). forall (a :: TYPE r).
                 HasCallStack => a
    undefined =  error "Prelude.undefined"

    -- | Used for compiler-generated error message;
    -- encoding saves bytes of string junk.
    absentErr :: a
    absentErr = errorWithoutStackTrace "Oops! The program has entered an `absent' argument!\n"