feat(lazy_list): implement ML-like lazy lists

We will use lazy lists to represent the set of solutions produced by the elaborator. The elaborator plugins will also use lazy lists.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/tests/util/CMakeLists.txt

@@ -49,3 +49,6 @@ add_test(exception ${CMAKE_CURRENT_BINARY_DIR}/exception)
 add_executable(bit_tricks bit_tricks.cpp)
 target_link_libraries(bit_tricks ${EXTRA_LIBS})
 add_test(bit_tricks ${CMAKE_CURRENT_BINARY_DIR}/bit_tricks)
+add_executable(lazy_list lazy_list.cpp)
+target_link_libraries(lazy_list ${EXTRA_LIBS})
+add_test(lazy_list ${CMAKE_CURRENT_BINARY_DIR}/lazy_list)

src/tests/util/lazy_list.cpp

@@ -0,0 +1,62 @@
+/*
+Copyright (c) 2013 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include <iostream>
+#include <utility>
+#include "util/test.h"
+#include "util/numerics/mpz.h"
+#include "util/pair.h"
+#include "util/lazy_list.h"
+using namespace lean;
+
+lazy_list<int> seq(int s) {
+    return lazy_list<int>(s, [=]() { return seq(s + 1); });
+}
+
+lazy_list<int> from(int begin, int step, int end) {
+    if (begin > end)
+        return lazy_list<int>();
+    else
+        return lazy_list<int>(begin, [=]() { return from(begin + step, step, end); });
+}
+
+lazy_list<mpz> fact_list_core(mpz i, mpz n) {
+    return lazy_list<mpz>(n, [=]() { return fact_list_core(i+1, n*(i+1)); });
+}
+
+lazy_list<mpz> fact_list() {
+    return fact_list_core(mpz(1), mpz(1));
+}
+
+template<typename T>
+lazy_list<T> first(unsigned sz, lazy_list<T> l) {
+    if (sz == 0 || !l) {
+        return lazy_list<T>();
+    } else {
+        return lazy_list<T>(head(l), [=]() { return first(sz - 1, tail(l)); });
+    }
+}
+
+template<typename T1, typename T2>
+lazy_list<std::pair<T1, T2>> zip(lazy_list<T1> const & l1, lazy_list<T2> const & l2) {
+    if (l1 && l2) {
+        return lazy_list<std::pair<T1, T2>>(mk_pair(head(l1), head(l2)), [=]() { return zip(tail(l1), tail(l2)); });
+    } else {
+        return lazy_list<std::pair<T1, T2>>();
+    }
+}
+
+int main() {
+    lazy_list<int> l(-10, from(10, 20, 100));
+    l = cons(-20, l);
+    for (auto c : l) {
+        std::cout << c << "\n";
+    }
+    for (auto c : first(30, zip(seq(1), fact_list()))) {
+        std::cout << c.first << " " << c.second << "\n";
+    }
+    return 0;
+}

src/util/lazy_list.h

@@ -0,0 +1,84 @@
+/*
+Copyright (c) 2013 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#pragma once
+#include "util/rc.h"
+#include "util/debug.h"
+
+namespace lean {
+/**
+   \brief ML-like lazy lists.
+*/
+template<typename T>
+class lazy_list {
+public:
+    class cell {
+        MK_LEAN_RC();
+        void dealloc() { delete this; }
+        T      m_head;
+    public:
+        cell(T const & h):m_rc(0), m_head(h) {}
+        virtual ~cell() {}
+        T const & head() const { return m_head; }
+        virtual lazy_list tail() const { return lazy_list(); }
+    };
+
+private:
+    class explicit_cell : public cell {
+        lazy_list m_tail;
+    public:
+        explicit_cell(T const & h, lazy_list const & l):cell(h), m_tail(l) {}
+        virtual ~explicit_cell() {}
+        virtual lazy_list tail() const { return m_tail; }
+    };
+
+    template<typename Tail>
+    class lazy_cell : public cell {
+        Tail m_tail;
+    public:
+        lazy_cell(T const & h, Tail const & t):cell(h), m_tail(t) {}
+        virtual ~lazy_cell() {}
+        virtual lazy_list tail() const { return m_tail(); }
+    };
+
+    cell * m_ptr;
+public:
+    lazy_list():m_ptr(nullptr) {}
+    lazy_list(T const & h):m_ptr(new cell(h)) { m_ptr->inc_ref(); }
+    lazy_list(T const & h, lazy_list const & t):m_ptr(new explicit_cell(h, t)) { m_ptr->inc_ref(); }
+    template<typename F>
+    lazy_list(T const & h, F const & f):m_ptr(new lazy_cell<F>(h, f)) { m_ptr->inc_ref(); }
+    lazy_list(lazy_list const & s):m_ptr(s.m_ptr) { if (m_ptr) m_ptr->inc_ref(); }
+    lazy_list(lazy_list&& s):m_ptr(s.m_ptr) { s.m_ptr = nullptr; }
+    ~lazy_list() { if (m_ptr) m_ptr->dec_ref(); }
+
+    lazy_list & operator=(lazy_list const & s) { LEAN_COPY_REF(list, s); }
+    lazy_list & operator=(lazy_list && s) { LEAN_MOVE_REF(list, s); }
+
+    operator bool() const { return m_ptr != nullptr; }
+
+    friend T const & head(lazy_list const & l) { lean_assert(l); return l.m_ptr->head(); }
+    friend lazy_list tail(lazy_list const & l) { lean_assert(l); return l.m_ptr->tail(); }
+    friend lazy_list cons(T const & h, lazy_list const & t) { return lazy_list(h, t); }
+
+    class iterator {
+        friend class lazy_list;
+        lazy_list m_it;
+        iterator(lazy_list const & it):m_it(it) {}
+    public:
+        iterator(iterator const & s):m_it(s.m_it) {}
+        iterator & operator++() { m_it = tail(m_it); return *this; }
+        iterator operator++(int) { iterator tmp(*this); operator++(); return tmp; }
+        bool operator==(iterator const & s) const { return m_it == s.m_it; }
+        bool operator!=(iterator const & s) const { return !operator==(s); }
+        T const & operator*() { lean_assert(m_it); return head(m_it); }
+        T const * operator->() { lean_assert(m_it); return &(head(m_it)); }
+    };
+
+    iterator begin() const { return iterator(*this); }
+    iterator end() const { return iterator(lazy_list()); }
+};
+}