add #occurs_free

[lambda.git] / exercises / assignment5.mdwn

diff --git a/exercises/assignment5.mdwn b/exercises/assignment5.mdwn
index f4955d9..dabc1a0 100644 (file)
--- a/exercises/assignment5.mdwn
+++ b/exercises/assignment5.mdwn
@@ -116,7 +116,7 @@ Choose one of these languages and write the following functions.
 
 6.  How would you use the function defined in problem 4 to enumerate a tree's fringe? (Don't worry about whether it comes out left-to-right or right-to-left.)
 
 
 6.  How would you use the function defined in problem 4 to enumerate a tree's fringe? (Don't worry about whether it comes out left-to-right or right-to-left.)
 

-7.  Write a recursive function to make a copy of a `color_tree` with the same structure and inner branch colors, but where the leftmost leaf is now labeled `0`, the second-leftmost leaf is now labeled `1`, and so on. (Here's a [[hint|assignment5 hint4]], if you need one.)
+7.  Write a recursive function to make a copy of a `color_tree` with the same structure and inner branch colors, but where the leftmost leaf is now labeled `0`, the second-leftmost leaf is now labeled `1`, and so on. (Here's a [[hint|assignment5 hint3]], if you need one.)

 
 8.  (More challenging.) Write a recursive function that makes a copy of a `color_tree` with the same structure and inner branch colors, but replaces each leaf label with the `int` that reports how many of that leaf's ancestors are labeled `Red`. For example, if we give your function a tree:
 
 
 8.  (More challenging.) Write a recursive function that makes a copy of a `color_tree` with the same structure and inner branch colors, but replaces each leaf label with the `int` that reports how many of that leaf's ancestors are labeled `Red`. For example, if we give your function a tree:
 


@@ -252,6 +252,7 @@ Again, we've left some gaps. (The use of `type` for the first line in Haskell an
 
 15. Choose one of these languages and fill in the gaps to complete the definition.
 
 
 15. Choose one of these languages and fill in the gaps to complete the definition.
 

+<a id="occurs_free"></a>

 16. Write a function `occurs_free` that has the following type:
 
         occurs_free : identifier -> lambda_term -> bool
 16. Write a function `occurs_free` that has the following type:
 
         occurs_free : identifier -> lambda_term -> bool


@@ -329,7 +330,7 @@ any type `α`, as long as your function is of type `α -> α` and you have a bas
         -- Or this:
         let sysf_true = (\y n -> y) :: Sysf_bool a
 
         -- 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:
+    Note that in both OCaml and Haskell code, the generalization `∀α` on the free type variable `α` is implicit. If you really want to, you can supply it explicitly in Haskell by saying:

 
         :set -XExplicitForAll
         let { sysf_true :: forall a. Sysf_bool a; ... }
 
         :set -XExplicitForAll
         let { sysf_true :: forall a. Sysf_bool a; ... }


@@ -408,7 +409,7 @@ Be sure to test your proposals with simple lists. (You'll have to `sysf_cons` up
         # k 1 true ;;
         - : int = 1
 
         # 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".