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Diffstat (limited to 'docs/users_guide/glasgow_exts.rst')
| -rw-r--r-- | docs/users_guide/glasgow_exts.rst | 43 |
1 files changed, 0 insertions, 43 deletions
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst index 3c951466c9..5cf3c4b9cc 100644 --- a/docs/users_guide/glasgow_exts.rst +++ b/docs/users_guide/glasgow_exts.rst @@ -15541,49 +15541,6 @@ the user must provide a type signature. :: foo :: [a] -> Int foo T = 5 -.. _multiple-complete-pragmas: - -Disambiguating between multiple ``COMPLETE`` pragmas ----------------------------------------------------- - -What should happen if there are multiple ``COMPLETE`` sets that apply to a -single set of patterns? Consider this example: :: - - data T = MkT1 | MkT2 | MkT2Internal - {-# COMPLETE MkT1, MkT2 #-} - {-# COMPLETE MkT1, MkT2Internal #-} - - f :: T -> Bool - f MkT1 = True - f MkT2 = False - -Which ``COMPLETE`` pragma should be used when checking the coverage of the -patterns in ``f``? If we pick the ``COMPLETE`` set that covers ``MkT1`` and -``MkT2``, then ``f`` is exhaustive, but if we pick the other ``COMPLETE`` set -that covers ``MkT1`` and ``MkT2Internal``, then ``f`` is *not* exhaustive, -since it fails to match ``MkT2Internal``. An intuitive way to solve this -dilemma is to recognize that picking the former ``COMPLETE`` set produces the -fewest number of uncovered pattern clauses, and thus is the better choice. - -GHC disambiguates between multiple ``COMPLETE`` sets based on this rationale. -To make things more formal, when the pattern-match checker requests a set of -constructors for some data type constructor ``T``, the checker returns: - -* The original set of data constructors for ``T`` -* Any ``COMPLETE`` sets of type ``T`` - -GHC then checks for pattern coverage using each of these sets. If any of these -sets passes the pattern coverage checker with no warnings, then we are done. If -each set produces at least one warning, then GHC must pick one of the sets of -warnings depending on how good the results are. The results are prioritized in -this order: - -1. Fewest uncovered clauses -2. Fewest redundant clauses -3. Fewest inaccessible clauses -4. Whether the match comes from the original set of data constructors or from a - ``COMPLETE`` pragma (prioritizing the former over the latter) - .. _overlap-pragma: ``OVERLAPPING``, ``OVERLAPPABLE``, ``OVERLAPS``, and ``INCOHERENT`` pragmas |