Just another update

Task/Forward-difference/D/forward-difference-1.d

@@ -1,20 +1,19 @@
-import std.stdio;
-
 T[] forwardDifference(T)(in T[] data, in int level) pure nothrow
-    in {
-        assert(level >= 0 && level < data.length);
-    } body {
-        //auto result = data.dup; // not nothrow
-        auto result = data ~ []; // slower
-        foreach (i; 0 .. level)
-            foreach (j, ref el; result[0 .. $ - i - 1])
-                el = result[j + 1] - el;
-        result.length -= level;
-        return result;
-    }
+in {
+    assert(level >= 0 && level < data.length);
+} body {
+    auto result = data.dup;
+    foreach (immutable i; 0 .. level)
+        foreach (immutable j, ref el; result[0 .. $ - i - 1])
+            el = result[j + 1] - el;
+    result.length -= level;
+    return result;
+}
 
 void main() {
-    auto data = [90.5, 47, 58, 29, 22, 32, 55, 5, 55, 73.5];
-    foreach (level; 0 .. data.length)
-        writeln(forwardDifference(data, level));
+    import std.stdio;
+
+    const data = [90.5, 47, 58, 29, 22, 32, 55, 5, 55, 73.5];
+    foreach (immutable level; 0 .. data.length)
+        forwardDifference(data, level).writeln;
 }

Task/Forward-difference/D/forward-difference-2.d

@@ -1,6 +1,6 @@
 import std.stdio, std.algorithm, std.range, std.array;
 
-auto forwardDifference(Range)(Range d, in int level) {
+auto forwardDifference(Range)(Range d, in int level) pure {
     foreach (immutable _; 0 .. level)
         d = d.zip(d.dropOne).map!(a => a[0] - a[1]).array;
     return d;

Task/Forward-difference/D/forward-difference-3.d

@@ -1,12 +1,12 @@
 import std.stdio;
 
-T[] forwardDifference(T)(T[] s, in int n) pure {
-    foreach (_; 0 .. n)
-        s[] -= s[1 .. $];
+T[] forwardDifference(T)(T[] s, in int n) pure nothrow @nogc {
+    foreach (immutable i; 0 .. n)
+        s[0 .. $ - i - 1] = s[1 .. $ - i] - s[0 .. $ - i - 1];
     return s[0 .. $ - n];
 }
 void main() {
     immutable A = [90.5, 47, 58, 29, 22, 32, 55, 5, 55, 73.5];
-    foreach (level; 0 .. A.length)
-        writeln(forwardDifference(A.dup, level));
+    foreach (immutable level; 0 .. A.length)
+        forwardDifference(A.dup, level).writeln;
 }

Task/Forward-difference/D/forward-difference-4.d

@@ -1,8 +1,10 @@
-import std.stdio, std.range;
-
 void main() {
+  import std.stdio, std.range;
+
   auto D = [90.5, 47, 58, 29, 22, 32, 55, 5, 55, 73.5];
-  auto R = recurrence!q{(a[n-1].dup[] -= a[n-1][1..$])[0..$-1]}(D);
-  foreach (di; take(R, D.length))
-    writeln(di);
+  writefln("%(%s\n%)",
+    recurrence!q{ (a[n - 1][0 .. $ - 1] =
+                   a[n - 1][1 .. $] -
+                   a[n - 1][0 .. $ - 1])[0 .. $] }(D)
+    .take(D.length));
 }