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>System.Random</TITLE
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>random-1.0.0.1: random number library</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
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>Index</A
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><FONT SIZE="6"
>System.Random</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>portable</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>stable</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>libraries@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>Random number generators
</A
></DT
><DD
><DL
><DT
><A HREF="#2"
>Standard random number generators
</A
></DT
><DT
><A HREF="#3"
>The global random number generator
</A
></DT
></DL
></DD
><DT
><A HREF="#4"
>Random values of various types
</A
></DT
><DT
><A HREF="#5"
>References
</A
></DT
></DL
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
><P
>This library deals with the common task of pseudo-random number
 generation. The library makes it possible to generate repeatable
 results, by starting with a specified initial random number generator,
 or to get different results on each run by using the system-initialised
 generator or by supplying a seed from some other source.
</P
><P
>The library is split into two layers: 
</P
><UL
><LI
> A core <EM
>random number generator</EM
> provides a supply of bits.
   The class <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
> provides a common interface to such generators.
   The library provides one instance of <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
>, the abstract
   data type <TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
>.  Programmers may, of course, supply their own
   instances of <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
>.
</LI
><LI
> The class <TT
><A HREF="System-Random.html#t%3ARandom"
>Random</A
></TT
> provides a way to extract values of a particular
   type from a random number generator.  For example, the <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AFloat"
>Float</A
></TT
>
   instance of <TT
><A HREF="System-Random.html#t%3ARandom"
>Random</A
></TT
> allows one to generate random values of type
   <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AFloat"
>Float</A
></TT
>.
</LI
></UL
><P
>This implementation uses the Portable Combined Generator of L'Ecuyer
 [<A HREF="System-Random.html#LEcuyer"
>System.Random</A
>] for 32-bit computers, transliterated by
 Lennart Augustsson.  It has a period of roughly 2.30584e18.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
>  <A HREF="#t%3ARandomGen"
>RandomGen</A
> g  <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="#v%3Anext"
>next</A
> :: g -&gt; (<A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>, g)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Asplit"
>split</A
> :: g -&gt; (g, g)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AgenRange"
>genRange</A
> :: g -&gt; (<A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>, <A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>)</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A HREF="#t%3AStdGen"
>StdGen</A
> </TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmkStdGen"
>mkStdGen</A
> :: <A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
> -&gt; <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AgetStdRandom"
>getStdRandom</A
> ::  (<A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
> -&gt; (a, <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
>)) -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AgetStdGen"
>getStdGen</A
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AsetStdGen"
>setStdGen</A
> :: <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
> -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AnewStdGen"
>newStdGen</A
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
>  <A HREF="#t%3ARandom"
>Random</A
> a  <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="#v%3ArandomR"
>randomR</A
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; (a, a) -&gt; g -&gt; (a, g)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Arandom"
>random</A
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; g -&gt; (a, g)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3ArandomRs"
>randomRs</A
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; (a, a) -&gt; g -&gt; [a]</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Arandoms"
>randoms</A
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; g -&gt; [a]</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3ArandomRIO"
>randomRIO</A
> :: (a, a) -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3ArandomIO"
>randomIO</A
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
>Random number generators
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
>  <A NAME="t:RandomGen"
><A NAME="t%3ARandomGen"
></A
></A
><B
>RandomGen</B
> g  <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
>The class <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
> provides a common interface to random number
 generators.
</P
><P
>Minimal complete definition: <TT
><A HREF="System-Random.html#v%3Anext"
>next</A
></TT
> and <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
>.
</P
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
>Methods</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A NAME="v:next"
><A NAME="v%3Anext"
></A
></A
><B
>next</B
> :: g -&gt; (<A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>, g)</TD
></TR
><TR
><TD CLASS="doc"
>The <TT
><A HREF="System-Random.html#v%3Anext"
>next</A
></TT
> operation returns an <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TT
> that is uniformly distributed
 in the range returned by <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> (including both end points),
 and a new generator.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:split"
><A NAME="v%3Asplit"
></A
></A
><B
>split</B
> :: g -&gt; (g, g)</TD
></TR
><TR
><TD CLASS="doc"
>The <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
> operation allows one to obtain two distinct random number
 generators. This is very useful in functional programs (for example, when
 passing a random number generator down to recursive calls), but very
 little work has been done on statistically robust implementations of
 <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
> ([<A HREF="System-Random.html#Burton"
>System.Random</A
>, <A HREF="System-Random.html#Hellekalek"
>System.Random</A
>]
 are the only examples we know of).
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:genRange"
><A NAME="v%3AgenRange"
></A
></A
><B
>genRange</B
> :: g -&gt; (<A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>, <A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
>)</TD
></TR
><TR
><TD CLASS="doc"
><P
>The <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> operation yields the range of values returned by
 the generator.
</P
><P
>It is required that:
</P
><UL
><LI
> If <TT
>(a,b) = <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> g</TT
>, then <TT
>a &lt; b</TT
>.
</LI
><LI
> <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> always returns a pair of defined <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TT
>s.
</LI
></UL
><P
>The second condition ensures that <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> cannot examine its
 argument, and hence the value it returns can be determined only by the
 instance of <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
>.  That in turn allows an implementation to make
 a single call to <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> to establish a generator's range, without
 being concerned that the generator returned by (say) <TT
><A HREF="System-Random.html#v%3Anext"
>next</A
></TT
> might have
 a different range to the generator passed to <TT
><A HREF="System-Random.html#v%3Anext"
>next</A
></TT
>.
</P
><P
>The default definition spans the full range of <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TT
>.
</P
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:RandomGen')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:RandomGen" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="2"
><A NAME="2"
>Standard random number generators
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A NAME="t:StdGen"
><A NAME="t%3AStdGen"
></A
></A
><B
>StdGen</B
>  </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
>The <TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
> instance of <TT
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
></TT
> has a <TT
><A HREF="System-Random.html#v%3AgenRange"
>genRange</A
></TT
> of at least 30 bits.
</P
><P
>The result of repeatedly using <TT
><A HREF="System-Random.html#v%3Anext"
>next</A
></TT
> should be at least as statistically
robust as the <EM
>Minimal Standard Random Number Generator</EM
> described by
[<A HREF="System-Random.html#Park"
>System.Random</A
>, <A HREF="System-Random.html#Carta"
>System.Random</A
>].
Until more is known about implementations of <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
>, all we require is
that <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
> deliver generators that are (a) not identical and
(b) independently robust in the sense just given.
</P
><P
>The <TT
><A HREF="../base/Text-Show.html#t%3AShow"
>Show</A
></TT
> and <TT
><A HREF="../base/Text-Read.html#t%3ARead"
>Read</A
></TT
> instances of <TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
> provide a primitive way to save the
state of a random number generator.
It is required that <TT
><TT
><A HREF="../base/Text-Read.html#v%3Aread"
>read</A
></TT
> (<TT
><A HREF="../base/Text-Show.html#v%3Ashow"
>show</A
></TT
> g) == g</TT
>.
</P
><P
>In addition, <TT
><A HREF="../base/Text-Read.html#v%3Aread"
>read</A
></TT
> may be used to map an arbitrary string (not necessarily one
produced by <TT
><A HREF="../base/Text-Show.html#v%3Ashow"
>show</A
></TT
>) onto a value of type <TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
>. In general, the <TT
><A HREF="../base/Text-Read.html#v%3Aread"
>read</A
></TT
>
instance of <TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
> has the following properties: 
</P
><UL
><LI
> It guarantees to succeed on any string. 
</LI
><LI
> It guarantees to consume only a finite portion of the string. 
</LI
><LI
> Different argument strings are likely to result in different results.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:StdGen')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:StdGen" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="../base/Text-Read.html#t%3ARead"
>Read</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Text-Show.html#t%3AShow"
>Show</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:mkStdGen"
><A NAME="v%3AmkStdGen"
></A
></A
><B
>mkStdGen</B
> :: <A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
> -&gt; <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="doc"
>The function <TT
><A HREF="System-Random.html#v%3AmkStdGen"
>mkStdGen</A
></TT
> provides an alternative way of producing an initial
generator, by mapping an <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TT
> into a generator. Again, distinct arguments
should be likely to produce distinct generators.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section2"
><A NAME="3"
><A NAME="3"
>The global random number generator
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
><A NAME="globalrng"
><A NAME="globalrng"
></A
></A
>
</P
><P
>There is a single, implicit, global random number generator of type
<TT
><A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TT
>, held in some global variable maintained by the <TT
><A HREF="../base/System-IO.html#t%3AIO"
>IO</A
></TT
> monad. It is
initialised automatically in some system-dependent fashion, for example, by
using the time of day, or Linux's kernel random number generator. To get
deterministic behaviour, use <TT
><A HREF="System-Random.html#v%3AsetStdGen"
>setStdGen</A
></TT
>.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:getStdRandom"
><A NAME="v%3AgetStdRandom"
></A
></A
><B
>getStdRandom</B
> ::  (<A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
> -&gt; (a, <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
>)) -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Uses the supplied function to get a value from the current global
random generator, and updates the global generator with the new generator
returned by the function. For example, <TT
>rollDice</TT
> gets a random integer
between 1 and 6:
</P
><PRE
>  rollDice :: IO Int
  rollDice = getStdRandom (randomR (1,6))
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:getStdGen"
><A NAME="v%3AgetStdGen"
></A
></A
><B
>getStdGen</B
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="doc"
>Gets the global random number generator.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:setStdGen"
><A NAME="v%3AsetStdGen"
></A
></A
><B
>setStdGen</B
> :: <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
> -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
>Sets the global random number generator.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:newStdGen"
><A NAME="v%3AnewStdGen"
></A
></A
><B
>newStdGen</B
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> <A HREF="System-Random.html#t%3AStdGen"
>StdGen</A
></TD
></TR
><TR
><TD CLASS="doc"
>Applies <TT
><A HREF="System-Random.html#v%3Asplit"
>split</A
></TT
> to the current global random generator,
 updates it with one of the results, and returns the other.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="4"
><A NAME="4"
>Random values of various types
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
>  <A NAME="t:Random"
><A NAME="t%3ARandom"
></A
></A
><B
>Random</B
> a  <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
>With a source of random number supply in hand, the <TT
><A HREF="System-Random.html#t%3ARandom"
>Random</A
></TT
> class allows the
programmer to extract random values of a variety of types.
</P
><P
>Minimal complete definition: <TT
><A HREF="System-Random.html#v%3ArandomR"
>randomR</A
></TT
> and <TT
><A HREF="System-Random.html#v%3Arandom"
>random</A
></TT
>.
</P
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
>Methods</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A NAME="v:randomR"
><A NAME="v%3ArandomR"
></A
></A
><B
>randomR</B
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; (a, a) -&gt; g -&gt; (a, g)</TD
></TR
><TR
><TD CLASS="doc"
>Takes a range <EM
>(lo,hi)</EM
> and a random number generator
 <EM
>g</EM
>, and returns a random value uniformly distributed in the closed
 interval <EM
>[lo,hi]</EM
>, together with a new generator. It is unspecified
 what happens if <EM
>lo&gt;hi</EM
>. For continuous types there is no requirement
 that the values <EM
>lo</EM
> and <EM
>hi</EM
> are ever produced, but they may be,
 depending on the implementation and the interval.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:random"
><A NAME="v%3Arandom"
></A
></A
><B
>random</B
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; g -&gt; (a, g)</TD
></TR
><TR
><TD CLASS="doc"
><P
>The same as <TT
><A HREF="System-Random.html#v%3ArandomR"
>randomR</A
></TT
>, but using a default range determined by the type:
</P
><UL
><LI
> For bounded types (instances of <TT
><A HREF="../base/Prelude.html#t%3ABounded"
>Bounded</A
></TT
>, such as <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AChar"
>Char</A
></TT
>),
   the range is normally the whole type.
</LI
><LI
> For fractional types, the range is normally the semi-closed interval
 <TT
>[0,1)</TT
>.
</LI
><LI
> For <TT
><A HREF="../integer/GHC-Integer.html#t%3AInteger"
>Integer</A
></TT
>, the range is (arbitrarily) the range of <TT
><A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TT
>.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:randomRs"
><A NAME="v%3ArandomRs"
></A
></A
><B
>randomRs</B
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; (a, a) -&gt; g -&gt; [a]</TD
></TR
><TR
><TD CLASS="doc"
>Plural variant of <TT
><A HREF="System-Random.html#v%3ArandomR"
>randomR</A
></TT
>, producing an infinite list of
 random values instead of returning a new generator.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:randoms"
><A NAME="v%3Arandoms"
></A
></A
><B
>randoms</B
> :: <A HREF="System-Random.html#t%3ARandomGen"
>RandomGen</A
> g =&gt; g -&gt; [a]</TD
></TR
><TR
><TD CLASS="doc"
>Plural variant of <TT
><A HREF="System-Random.html#v%3Arandom"
>random</A
></TT
>, producing an infinite list of
 random values instead of returning a new generator.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:randomRIO"
><A NAME="v%3ArandomRIO"
></A
></A
><B
>randomRIO</B
> :: (a, a) -&gt; <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>A variant of <TT
><A HREF="System-Random.html#v%3ArandomR"
>randomR</A
></TT
> that uses the global random number generator
 (see <A HREF="System-Random.html#globalrng"
>System.Random</A
>).
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:randomIO"
><A NAME="v%3ArandomIO"
></A
></A
><B
>randomIO</B
> :: <A HREF="../base/System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>A variant of <TT
><A HREF="System-Random.html#v%3Arandom"
>random</A
></TT
> that uses the global random number generator
 (see <A HREF="System-Random.html#globalrng"
>System.Random</A
>).
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:Random')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:Random" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../ghc-prim/GHC-Types.html#t%3AChar"
>Char</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../ghc-prim/GHC-Types.html#t%3ADouble"
>Double</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../ghc-prim/GHC-Types.html#t%3AFloat"
>Float</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../ghc-prim/GHC-Types.html#t%3AInt"
>Int</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="System-Random.html#t%3ARandom"
>Random</A
> <A HREF="../integer/GHC-Integer.html#t%3AInteger"
>Integer</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="5"
><A NAME="5"
>References
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>1. FW <A NAME="Burton"
><A NAME="Burton"
></A
></A
> Burton and RL Page, <EM
>Distributed random number generation</EM
>,
Journal of Functional Programming, 2(2):203-212, April 1992.
</P
><P
>2. SK <A NAME="Park"
><A NAME="Park"
></A
></A
> Park, and KW Miller, /Random number generators -
good ones are hard to find/, Comm ACM 31(10), Oct 1988, pp1192-1201.
</P
><P
>3. DG <A NAME="Carta"
><A NAME="Carta"
></A
></A
> Carta, /Two fast implementations of the minimal standard
random number generator/, Comm ACM, 33(1), Jan 1990, pp87-88.
</P
><P
>4. P <A NAME="Hellekalek"
><A NAME="Hellekalek"
></A
></A
> Hellekalek, <EM
>Don\'t trust parallel Monte Carlo</EM
>,
Department of Mathematics, University of Salzburg,
<A HREF="http://random.mat.sbg.ac.at/~peter/pads98.ps"
>http://random.mat.sbg.ac.at/~peter/pads98.ps</A
>, 1998.
</P
><P
>5. Pierre <A NAME="LEcuyer"
><A NAME="LEcuyer"
></A
></A
> L'Ecuyer, /Efficient and portable combined random
number generators/, Comm ACM, 31(6), Jun 1988, pp742-749.
</P
><P
>The Web site <A HREF="http://random.mat.sbg.ac.at/"
>http://random.mat.sbg.ac.at/</A
> is a great source of information.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="botbar"
>Produced by <A HREF="http://www.haskell.org/haddock/"
>Haddock</A
> version 2.4.2</TD
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