examples/old/Markus/higher_order_functions.spl

   1 // Depending on your design decisions this might or might not compile.
   2 // In our course we don't require higher-order functions.
   3 // In fact, our grammar doesn't even allow function types in type signatures.
   4 // Anyway, this could be an idea for assignment 4.
   5
   6 map(f, list)
   7 // This doesn't parse according to our grammar
   8 // :: (a -> b) [a] -> [b]
   9 {
  10   if( isEmpty(list) )
  11     return [];
  12   else
  13     return f(list.hd) : map(f, list.tl);
  14 }
  15
  16 foldl(f, z, list)
  17 {
  18   if( isEmpty(list) )
  19     return z;
  20   else
  21     return foldl(f, f(z, list.hd), list.tl);
  22 }
  23
  24 // Some operators wrapped in a function.
  25 plus(x, y) { return x + y; }
  26 and(b, c) { return b && c; }
  27 ge18(x) { return x >= 18; }
  28
  29 // Sums all the elements of an Int list.
  30 sum(list) { return foldl(plus, 0, list); }
  31
  32 // Checks whether all elements in a list of Booleans are True
  33 all(list) { return foldl(and, True, list); }
  34
  35 // Checks whether in a list of numbers everybody is older than 18
  36 allOlderThan18(list) { return all(map(ge18, list)); }
  37
  38 main() { return; }