From: jim
Date: Sun, 1 Mar 2015 21:54:02 +0000 (-0500)
Subject: tweak types for list encoding
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=1c184042007c8578ddabcc5590d372b1e469cb91
tweak types for list encoding
---
diff --git a/exercises/_assignment5.mdwn b/exercises/_assignment5.mdwn
index e081c671..7765a7e7 100644
--- a/exercises/_assignment5.mdwn
+++ b/exercises/_assignment5.mdwn
@@ -351,10 +351,10 @@ any type `Î±`, as long as your function is of type `Î± -> Î±` and you have a bas
Consider the following list type, specified using OCaml or Haskell datatypes:
(* OCaml *)
- type ('a) my_list = Nil | Cons of 'a * 'a my_list
+ type ('t) my_list = Nil | Cons of 't * 't my_list
-- Haskell
- data My_list a = Nil | Cons a (My_list a) deriving (Show)
+ data My_list t = Nil | Cons t (My_list t) deriving (Show)
We can encode that type into System F in terms of its right-fold, just as we did in the untyped Lambda Calculus, like this:
@@ -364,27 +364,27 @@ We can encode that type into System F in terms of its right-fold, just as we did
As with `Nat`s, the natural recursion on the type is built into our encoding of it.
-There is some awkwardness here, because System F doesn't have any parameterized types like OCaml's `('a) list` or Haskell's `[a]`. For those, we need to use a more complex system called System F_{Ï}. System F *can* already define a more polymorphic list type:
+There is some awkwardness here, because System F doesn't have any parameterized types like OCaml's `('t) my_list` or Haskell's `My_list t`. For those, we need to use a more complex system called System F_{Ï}. System F *can* already define a more polymorphic list type:
- list â¡ âÎ². âÎ±. (Î² -> Î± -> Î±) -> Î± -> Î±
+ list â¡ âÏ. âÎ±. (Ï -> Î± -> Î±) -> Î± -> Î±
But this is more awkward to work with, because for functions like `map` we want to give them not just the type:
- (S -> T) -> list -> list
+ (T -> S) -> list -> list
but more specifically, the type:
- (S -> T) -> list [S] -> list [T]
+ (T -> S) -> list [T] -> list [S]
Yet we haven't given ourselves the capacity to talk about `list [S]` and so on as a type in System F. Hence, I'll just use the more clumsy, ad hoc specification of `map`'s type as:
- (S -> T) -> list_S -> list_T
+ (T -> S) -> list_T -> list_S
-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 `('a) sysf_list` or `Sysf_list a`.)
+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.