As response to both Aaron and Duncan,
foo 0 = ... foo n = ...
And what about the negative numbers? For negative numbers the second equation matches, which in 90% of all cases in practise has never been written for them. Aaron's Ackerman function disappears in infinite recursion... Besides, what is ack 0.5 0.5? The use of n+k patterns, but also the definition pattern above wrongly lead programmers to believe that they are dealing with natural numbers. There is no nice primitive recursion for integers. Even worse, without a type signature restricting its type, foo will be defined for all numeric types. For Float or Rational it makes hardly any sense. If Haskell had a type for natural numbers I'd be in favour of n+k and k patterns (working only for this type, not any other numerical type). Using primitive recursion on integers or even arbitrary numbers is misleading. You can teach primitive recursion nicely for algebraic data types, because the recursive pattern of the function definition follows the recursive pattern of the type definition. With respect to tools of which Hat is one example: If it is hard to build tools, then less tools will be built. Compare the number of tools for Scheme with those for Haskell. Most tools grow out of student projects or research projects; these have rather limited resources. Ciao, Olaf