[project @ 2002-06-05 13:30:52 by simonpj]

[ghc-hetmet.git] / ghc / docs / users_guide / vs_haskell.sgml

diff --git a/ghc/docs/users_guide/vs_haskell.sgml b/ghc/docs/users_guide/vs_haskell.sgml
index 87c6b9c..f9e8481 100644 (file)
--- a/ghc/docs/users_guide/vs_haskell.sgml
+++ b/ghc/docs/users_guide/vs_haskell.sgml
@@ -221,6 +221,19 @@ main = print (array (1,1) [(1,2), (1,3)])</programlisting>
          writing <literal>0xffffffff :: Int</literal> preserves the
          bit-pattern in the resulting <literal>Int</literal>.</para>
 
+
+          <para>Negative literals, such as <literal>-3</literal>, are
+             specified by (a careful reading of) the Haskell Report as 
+             meaning <literal>Prelude.negate (Prelude.fromInteger 3)</literal>.
+            So <literal>-2147483648</literal> means <literal>negate (fromInteger 2147483648)</literal>.
+            Since <literal>fromInteger</literal> takes the lower 32 bits of the representation,
+            <literal>fromInteger (2147483648::Integer)</literal>, computed at type <literal>Int</literal> is
+            <literal>-2147483648::Int</literal>.  The <literal>negate</literal> operation then
+            overflows, but it is unchecked, so <literal>negate (-2147483648::Int)</literal> is just
+            <literal>-2147483648</literal>.  In short, one can write <literal>minBound::Int</literal> as
+            a literal with the expected meaning (but that is not in general guaranteed.
+             </para>
+
          <para>The <literal>fromIntegral</literal> function also
          preserves bit-patterns when converting between the sized
          integral types (<literal>Int8</literal>,