From 0544b52f1aa5cec4827ee7bd8a24efb336c7e5e0 Mon Sep 17 00:00:00 2001
From: Nicolas Boulenguez <nicolas.boulenguez@free.fr>
Date: Tue, 7 May 2019 22:59:57 +0200
Subject: [PATCH] Draft exercise suggesting native implementations of some
 built-in functions.

---
 docs/exercise.md       | 49 ++++++++++++++++++++++++++++++++++++++++++
 examples/exercises.mal | 48 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+)
 create mode 100644 docs/exercise.md
 create mode 100644 examples/exercises.mal

diff --git a/docs/exercise.md b/docs/exercise.md
new file mode 100644
index 00000000..6e811de2
--- /dev/null
+++ b/docs/exercise.md
@@ -0,0 +1,49 @@
+Once you have a working implementation, you may want to implement
+parts of the process inside the MAL language itself. This has no other
+purpose than learning the MAL language. Once it exists, a built-in
+implementation will always be more efficient than a native
+implementation. Also, the functions described in MAL process are
+selected for educative purposes, so portability accross
+implementations does not matter much.
+
+You may easily check your answers by passing them directly to the
+interpreter. They will hide the built-in functions carrying the same
+names, and the usual tests (with REGRESS=1) will check them. The
+`runtest.py` script provide a convenient command-line parameter to
+pass a command like 'load-file' before running the testsuite.
+
+Some solutions are given in the `examples` directory. Feel free to
+submit new solutions, or new exercises.
+
+
+- Implement the following functions with other built-in functions.
+  - `nil?`, `true?` and `false?`
+  - `empty?`
+  - `sequential?`
+
+- Implement `>`, `<=` and `>=` with `<`.
+
+- Implement the following non-recursive functions.
+  - `hash-map`
+  - `list`
+  - `prn`
+  - `swap!`
+
+- Implement `map` with a recursion.
+
+- Implement the `do` special as a non-recursive function. The special
+  form will hide your implementation, so in order to test it, you will
+  need to give it another name and adapt the test accordingly.
+
+- Implement `let*` as a macro that uses `fn*` and recursionn. The same
+  remark applies.
+
+- Implement `apply` as a macro.
+
+- Implement maps using lists.
+  FIXME: Is dissoc use anywhere? It makes this implememtation and the
+  process more complex.
+
+- Implement quoting within MAL.
+
+- Implement macros within MAL.
diff --git a/examples/exercises.mal b/examples/exercises.mal
new file mode 100644
index 00000000..6a6606fe
--- /dev/null
+++ b/examples/exercises.mal
@@ -0,0 +1,48 @@
+;; These are the answers to the questions in ../docs/exercise.md.
+
+(def! nil?   (fn* [x] (= x nil  )))
+(def! true?  (fn* [x] (= x true )))
+(def! false? (fn* [x] (= x false)))
+
+(def! empty? (fn* [xs] (= 0 (count xs))))
+
+(def! sequential? (fn* [x] (if (list? x) true (if (vector? x) true false))))
+
+(def! >  (fn* [a b]     (< b a)            ))
+(def! <= (fn* [a b] (if (< b a) false true)))
+(def! >= (fn* [a b] (if (< a b) false true)))
+
+(def! hash-map (fn* [& xs] (apply assoc {} xs)))
+(def! list (fn* [& xs] xs))
+(def! prn (fn* [& xs] (println (apply pr-str xs))))
+(def! swap! (fn* [a f & xs] (reset! a (apply f (deref a) xs))))
+
+(def! map
+  (fn* [f xs]
+    (if (empty? xs)
+      ()
+      (cons (f (first xs)) (map f (rest xs))))))
+
+(def! do2 (fn* [& xs] (nth xs (- (count xs) 1))))
+
+(defmacro! let2
+  ;; Must be a macro because the first argument must not be evaluated.
+  (fn* [binds form]
+    (if (empty? binds)
+      form
+      ;;  This let* increases the readability, but the values could
+      ;;  easily be replaced below.
+      (let* [key  (nth binds 0)
+             val  (nth binds 1)
+             more (rest (rest binds))]
+        `((fn* [~key] (let2 ~more ~form)) ~val)))))
+
+(defmacro! apply
+  (fn* [& xs]
+    (;; Rewrite (f a b [c d]) to (f a b c d).
+     (def! rec
+       (fn* [lst]
+         (if (= 1 (count lst))
+           (first lst)              ; last argument must be a sequence
+           (cons (first lst) (rec (rest lst))))))
+     xs)))