-- Or this:
let sysf_true = (\y n -> y) :: Sysf_bool a
- Note that in both OCaml and the Haskell code, the generalization `∀'a` on the free type variable `'a` is implicit. If you really want to, you can supply it explicitly in Haskell by saying:
-
- :set -XExplicitForAll
+ :set -XExplicitForAll
let { sysf_true :: forall a. Sysf_bool a; ... }
-- or
let { sysf_true :: forall a. a -> a -> a; ... }
= λf:T -> S. λxs:list. xs [T] [list [S]] (λx:T. λys:list [S]. cons [S] (f x) ys) (nil [S])
-->
+*Update: Never mind, don't bother with the next three questions. They proved to be more difficult to implement in OCaml than we expected. Here is [[some explanation|assignment5 hint4]].*
+
19. Convert this list encoding and the `map` function to OCaml or Haskell. Again, call the type `sysf_list`, and the functions `sysf_nil`, `sysf_cons`, and `sysf_map`, to avoid collision with the names for native lists and functions in these languages. (In OCaml and Haskell you *can* say `('t) sysf_list` or `Sysf_list t`.)
20. Also give us the type and definition for a `sysf_head` function. Think about what value to give back if its argument is the empty list. It might be cleanest to use the `option`/`Maybe` technique explored in questions 1--2, but for this assignment, just pick a strategy, no matter how clunky.
# k 1 true ;;
- : int = 1
- If you can't understand how one term can have several types, recall our discussion in this week's notes of "principal types". (WHERE?)
+ If you can't understand how one term can have several types, recall our discussion in this week's notes of "principal types".