.. _negative-literals:

Negative literals
-----------------

.. extension:: NegativeLiterals
    :shortdesc: Enable support for negative literals.

    :since: 7.8.1

    Enable negative numeric literals.

The literal ``-123`` is, according to Haskell98 and Haskell 2010,
two tokens, a unary minus (``-``) and the number 123, and is
desugared as ``negate (fromInteger 123)``. The language extension
:extension:`NegativeLiterals` causes it to be treated as a single
token and desugared as ``fromInteger (-123)``.

This can be useful when the positive and negative range of a numeric
data type don't match up. For example, in 8-bit arithmetic -128
is representable, but +128 is not. So ``negate (fromInteger 128)``
will elicit an unexpected integer-literal-overflow message.

Whitespace can be inserted, as in ``- 123``, to force interpretation
as two tokens.

In 9.0, the behavior of this extension changed, and now we require that a negative literal must not be preceded by a closing token (see
`GHC Proposal #229 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst>`__
for the definition of a closing token). In other words, we parse ``f -123`` as ``f (-123)``, but ``x-123`` as ``(-) x
123``. Before this amendment, :extension:`NegativeLiterals` caused ``x-123`` to be parsed as ``x(-123)``.

:extension:`NegativeLiterals` is a subset of :extension:`LexicalNegation`. That
is, enabling both of those extensions has the same effect as enabling
:extension:`LexicalNegation` alone.