From: Chris Barker <barker@kappa.linguistics.fas.nyu.edu>
Date: Tue, 26 Oct 2010 15:23:33 +0000 (-0400)
Subject: hw changes
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=90294e766ccb45391a5d5e9909a0720ed92cca60

hw changes
---

diff --git a/assignment5.mdwn b/assignment5.mdwn
index 4a4e06d2..cc714e90 100644
--- a/assignment5.mdwn
+++ b/assignment5.mdwn
@@ -1,10 +1,10 @@
 Assignment 5
 
-Types and OCAML
+Types and OCaml
 ---------------
 
 0. Recall that the S combinator is given by \x y z. x z (y z).
-   Give two different typings for this function in OCAML.
+   Give two different typings for this function in OCaml.
    To get you started, here's one typing for K:
 
     # let k (y:'a) (n:'b) = y;;
@@ -13,7 +13,7 @@ Types and OCAML
     - : int = 1
 
 
-1. Which of the following expressions is well-typed in OCAML?  
+1. Which of the following expressions is well-typed in OCaml?  
    For those that are, give the type of the expression as a whole.
    For those that are not, why not?
 
@@ -72,8 +72,8 @@ Types and OCAML
 The following expression is an attempt to make explicit the
 behavior of `if`-`then`-`else` explored in the previous question.
 The idea is to define an `if`-`then`-`else` expression using 
-other expression types.  So assume that "yes" is any OCAML expression,
-and "no" is any other OCAML expression (of the same type as "yes"!),
+other expression types.  So assume that "yes" is any OCaml expression,
+and "no" is any other OCaml expression (of the same type as "yes"!),
 and that "bool" is any boolean.  Then we can try the following:
 "if bool then yes else no" should be equivalent to
 
@@ -143,17 +143,17 @@ Baby monads
       match x with None -> None | Some n -> f n;;
 
 
-Booleans, Church numbers, and Church lists in OCAML
+Booleans, Church numbers, and Church lists in OCaml
 ---------------------------------------------------
 
 These questions adapted from web materials written by some smart dude named Acar.
 The idea is to get booleans, Church numbers, "Church" lists, and
-binary trees working in OCAML.
+binary trees working in OCaml.
 
    Recall from class System F, or the polymorphic λ-calculus.
 
-    τ ::= α | τ1 → τ2 | ∀α. τ
-    e ::= x | λx:τ. e | e1 e2 | Λα. e | e [τ ]
+    τ ::= 'α | τ1 → τ2 | ∀'α. τ | c
+    e ::= x | λx:τ. e | e1 e2 | Λ'α. e | e [τ ]
 
    Recall that bool may be encoded as follows:
 
@@ -180,8 +180,13 @@ binary trees working in OCAML.
    encoding above, the result of that iteration can be any type α, as long as you have a base element z : α and
    a function s : α → α.
 
-   **Excercise**: get booleans and Church numbers working in OCAML,
-     including OCAML versions of bool, true, false, zero, succ, add.
+   **Excercise**: get booleans and Church numbers working in OCaml,
+     including OCaml versions of bool, true, false, zero, succ, and pred.
+     It's especially useful to do a version of pred, starting with one
+     of the (untyped) versions available in the lambda library
+     accessible from the main wiki page.  The point of the excercise
+     is to do these things on your own, so avoid using the built-in
+     OCaml booleans and list predicates.
 
    Consider the following list type:
 
@@ -195,21 +200,15 @@ binary trees working in OCAML.
 
    As with nats, recursion is built into the datatype.
 
-   We can write functions like map:
+   We can write functions like head, isNil, and map:
 
     map : (σ → τ ) → σ list → τ list
-      := λf :σ → τ. λl:σ list. l [τ list] nilτ (λx:σ. λy:τ list. consτ (f x) y
 
-   **Excercise** convert this function to OCAML.  Also write an `append` function.
-   Test with simple lists.
+   We've given you the type for map, you only need to give the term.
 
-   Consider the following simple binary tree type:
+   With regard to `head`, think about what value to give back if the
+   argument is the empty list.  Ultimately, we might want to make use
+   of our `'a option` technique, but for this assignment, just pick a
+   strategy, no matter how clunky. 
 
-    type ’a tree = Leaf | Node of ’a tree * ’a * ’a tree
-
-   **Excercise**
-   Write a function `sumLeaves` that computes the sum of all the
-   leaves in an int tree.
-
-   Write a function `inOrder` : τ tree → τ list that computes the in-order traversal of a binary tree. You
-   may assume the above encoding of lists; define any auxiliary functions you need.
+   Please provide both the terms and the types for each item.