[Move foldr, foldl explanationny bits around.
Bryan O'Sullivan <bos@serpentine.com>**20070919224457] {
hunk ./en/ch04-fp.xml 976
-	out each step in the evaluation of <code>niceSum
-	  [1,2,3]</code>.</para>
-      <orderedlist>
-	<listitem>
-<literallayout>step 0                   (1:[2,3]) == step (0 + 1)             [2,3]</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>step (0 + 1)             (2:[3])   == step ((0 + 1) + 2)       [3]</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>step ((0 + 1) + 2)       [3]       == step (((0 + 1) + 2) + 3) []</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>step (((0 + 1) + 2) + 3) []        == (((0 + 1) + 2) + 3)</literallayout>
-	</listitem>
-      </orderedlist>
+	out each step in the evaluation of <code>step</code> function
+	when we call <code>niceSum [1,2,3]</code>.</para>
+
+      &Fold.hs:foldl.expand;
hunk ./en/ch04-fp.xml 1085
-      <para>Let's follow the same manual expansion steps with
+      <para>Let's follow the same manual evaluation process with
hunk ./en/ch04-fp.xml 1090
-      <orderedlist>
-	<listitem>
-<literallayout>              step (1:[2,3])          == 1 +           step [2,3]</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>1 +           step (2:[3])            == 1 + (2 +      step [3])</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>1 + (2 +      step [3])               == 1 + (2 + (3 + step []))</literallayout>
-	</listitem>
-	<listitem>
-<literallayout>1 + (2 + (3 + step []))               == 1 + (2 + (3 + 0))</literallayout>
-	</listitem>
-      </orderedlist>
+      &Fold.hs:foldr.expand;
hunk ./en/ch04-fp.xml 1106
-      <literallayout>1 : 2 : 3 : []
-1 + 2 + 3 + 0
-</literallayout>
+      &Fold.hs:foldr.sub;
hunk ./en/ch04-fp.xml 1157
-      <note>
+      <tip>
+	<title>Understanding foldl in terms of foldr</title>
+
hunk ./en/ch04-fp.xml 1166
-      </note>
+
+	<para>All you'll need to do is follow the same manual
+	  evaluation steps as we did to see what
+	  <function>foldl</function> and <function>foldr</function>
+	  were really doing.</para>
+      </tip>
hunk ./en/ch04-fp.xml 1175
-	<function>foldr</function> is <quote>more basic than</quote>
+	<function>foldr</function> is <quote>more basic</quote> than
hunk ./en/ch04-fp.xml 1186
-      <para id="x_H9">Another useful way to think about the way
-	<function>foldr</function> works is that it
-	<emphasis>transforms</emphasis> its input list.  Its first two
-	arguments are <quote>what to do with each head/tail element of
-	  the list</quote>, and <quote>what to substitute at the end
-	  of the list</quote>.</para>
+      <para id="x_H9">Returning to our earlier intuitive explanation
+	of what <function>foldr</function> does, another useful way to
+	think about it is that it <emphasis>transforms</emphasis> its
+	input list.  Its first two arguments are <quote>what to do
+	  with each head/tail element of the list</quote>, and
+	<quote>what to substitute for the end of the
+	  list</quote>.</para>
hunk ./en/ch04-fp.xml 1222
-      <para id="x_N9">Now that we can think in terms of transforming a list, it
-	becomes easier for us to reason about operations like summing a
-	list.  We replace the empty list with zero, which becomes our
-	first accumulator value.  We then apply the
-	<function>(+)</function> function to each element of the list
-	and an accumulator value, to give a new accumulator
-	value.</para>
-
-      &fold.ghci:sum;
-
-      <para id="x_O9">Knowing that <function>foldr</function> transforms a list
-	from right to left, we can <quote>unroll</quote> the
-	application of it by hand to see the intermediate accumulators
-	that it produces.</para>
-
-      &fold.ghci:sum.steps;
+      <para>Here, we're replacing each list constructor with another
+	list constructor, but we're replacing the empty list with the
+	list we want to append onto the end of our first list.</para>
hunk ./examples/ch04/Fold.hs 58
+{-- snippet foldl.expand --}
+-- step 0                   (1:2:3:[]) == step (0 + 1)             (2:3:[])
+-- step (0 + 1)             (2:3:[])   == step ((0 + 1) + 2)       (3:[])
+-- step ((0 + 1) + 2)       [3]        == step (((0 + 1) + 2) + 3) []
+-- step (((0 + 1) + 2) + 3) []         ==      (((0 + 1) + 2) + 3)
+{-- /snippet foldl.expand --}
+
+{-- snippet foldr.expand --}
+--               step (1:2:3:[])         == 1 +           step (2:3:[])
+-- 1 +           step (2:3:[])            == 1 + (2 +     step (3:[])
+-- 1 + (2 +      step [3])               == 1 + (2 + (3 + step []))
+-- 1 + (2 + (3 + step []))               == 1 + (2 + (3 + 0))
+{-- /snippet foldr.expand --}
+
+{-- snippet foldr.sub --}
+-- 1 : 2 : 3 : []
+-- 1 + 2 + 3 + 0
+{-- /snippet foldr.sub --}
hunk ./examples/ch04/fold.ghci 1
-:m +Fold.hs
+:load Fold.hs
hunk ./examples/ch04/fold.ghci 12
---# sum
-
-foldr (+) 0 [1,2,3]
-
---# sum.steps
-
-foldr (+) 0 []
-foldr (+) 0 [3]
-foldr (+) 0 [2,3]
-foldr (+) 0 [1,2,3]
-
}