[Noted functions, etc.
John Goerzen <jgoerzen@complete.org>**20070920063201] {
hunk ./en/ch13-data.xml 372
-    <title>Association Lists</title>
+    <title>Association Lists and Maps</title>
hunk ./en/ch13-data.xml 482
+
+  <sect1 id="hs.data.funcs">
+    <title>Functions Are Data, Too</title>
+    <para>
+      Back in the beginning of the chapter, we reminded you that Haskell isn't
+      object-oriented.  Part of Haskell's power is the ease with which you
+      can create and manipulate functions with it.  Let's take a look at a
+      record that stores a function as one of its fields:
+    </para>
+    &funcrecs.hs:all;
+    <para>
+      Notice the type of the <literal>colorCalc</literal> field: it's a
+      function.  It takes an <literal>Int</literal> and returns a tuple of
+      <literal>(CuscomColor, Int)</literal>.  We create two
+      <literal>FuncRec</literal> records: <literal>plus5</literal> and
+      <literal>always0</literal>.  Notice that the
+      <literal>colorCalc</literal> for both of them will always return the
+      color purple.  <literal>FuncRec</literal> itself has no field to store
+      the color in, yet that value somehow becomes part of the function
+      itself.  This is called a <emphasis>closure</emphasis>.  Let's play
+      with this a bit:
+    </para>
+    &funcrecs.ghci:all;
+    <para>
+      That worked well enough, but you might be wondering how to do something
+      more advanced such as making a piece of data available multiple places.
+      A type construction function can be helpful.  Here's an example:
+    </para>
+    &funcrecs2.hs:all;
+    <para>
+      Here we have a function called <literal>mkFuncRec</literal> that
+      takes a &String; and another function as parameters, and returns
+      a new <literal>FuncRec</literal> record.  Notice how both parameters to
+      <literal>mkFuncRec</literal> are used multiple places.  Let's try it
+      out:
+    </para>
+    &funcrecs2.ghci:all;
+    Notice the creation of <literal>plus5a</literal>.  We changed the
+    <literal>name</literal> field, but not the <literal>namedCalc</literal>
+    field.  That's why <literal>name</literal> has the new name, but
+    <literal>namedCalc</literal> still returns the name that was passed to
+    <literal>mkFuncRec</literal>; it doesn't change unless we explicitly
+    change it.
+  </sect1>
+
addfile ./examples/ch13/funcrecs.ghci
hunk ./examples/ch13/funcrecs.ghci 1
+--# all
+:l funcrecs.hs
+:t plus5
+name plus5
+:t colorCalc plus5
+(colorCalc plus5) 7
+:t colorCalc always0
+(colorCalc always0) 7
+
addfile ./examples/ch13/funcrecs.hs
hunk ./examples/ch13/funcrecs.hs 1
+{-- snippet all --}
+data CustomColor =
+  CustomColor {red :: Int,
+               green :: Int,
+               blue :: Int}
+  deriving (Eq, Show, Read)
+
+data FuncRec =
+    FuncRec {name :: String,
+             colorCalc :: Int -> (CustomColor, Int)}
+
+plus5func color x = (color, x + 5)
+purple = CustomColor 255 0 255
+
+plus5 = FuncRec {name = "plus5", colorCalc = plus5func purple}
+always0 = FuncRec {name = "always0", colorCalc = \_ -> (purple, 0)}
+{-- /snippet all --}
+
addfile ./examples/ch13/funcrecs2.ghci
hunk ./examples/ch13/funcrecs2.ghci 1
+--# all
+:l funcrecs2.hs
+:t plus5
+name plus5
+(calc plus5) 5
+(namedCalc plus5) 5
+let plus5a = plus5 {name = "PLUS5A"}
+name plus5a
+(namedCalc plus5a) 5
addfile ./examples/ch13/funcrecs2.hs
hunk ./examples/ch13/funcrecs2.hs 1
+{-- snippet all --}
+data FuncRec =
+    FuncRec {name :: String,
+             calc :: Int -> Int,
+             namedCalc :: Int -> (String, Int)}
+
+mkFuncRec :: String -> (Int -> Int) -> FuncRec
+mkFuncRec name calcfunc =
+    FuncRec {name = name,
+             calc = calcfunc,
+             namedCalc = \x -> (name, calcfunc x)}
+
+plus5 = mkFuncRec "plus5" (+ 5)
+always0 = mkFuncRec "always0" (\_ -> 0)
+{-- /snippet all --}
+
}