1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
//! List "cons cell" data type and accompanying iterator types.
use std::fmt;

use crate::Value;

/// A Lisp "cons cell".
///
/// A cons cell is similiar to a two-element tuple in Rust. Its fields
/// are traditionally called `car` and `cdr`, for obscure historical
/// reasons. Both the `car` and the `cdr` field can hold any `Value`,
/// including other cons cells.
///
/// This data type is used to represent singly-linked lists, by
/// forming a chain of cons cells where the list element is kept in
/// the `car` field, and the `cdr` field either points to the next
/// cons cell, or terminates the list with any other value. Usually,
/// that terminator value is `Value::Null`, also referred to as the
/// empty list. If any other terminating value is used, the resulting
/// linked list is referred to as "dotted", or "improper" list.
///
/// The `Cons` data type provides some utility function for the
/// singly-linked list use case, such as iterating through the list or
/// converting the list to a vector. To account for the possibility of
/// dotted lists, the iterators and vector conversion functions have
/// slightly unusual types.
///
/// The most natural way to traverse a singly linked list is probably by using
/// the `list_iter` method.
#[derive(PartialEq, Clone)]
pub struct Cons {
    inner: Box<(Value, Value)>,
}

impl fmt::Debug for Cons {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "({:?} . {:?})", self.car(), self.cdr())
    }
}

impl Cons {
    /// Constructs a new cons cell from two values.
    pub fn new<T, U>(car: T, cdr: U) -> Self
    where
        T: Into<Value>,
        U: Into<Value>,
    {
        Cons {
            inner: Box::new((car.into(), cdr.into())),
        }
    }

    /// Returns a reference to the value in the `car` field.
    pub fn car(&self) -> &Value {
        &self.inner.0
    }

    /// Returns a mutable reference to the value in the `car` field.
    pub fn car_mut(&mut self) -> &mut Value {
        &mut self.inner.0
    }

    /// Sets the `car` field.
    pub fn set_car(&mut self, car: impl Into<Value>) {
        self.inner.0 = car.into()
    }

    /// Returns a reference to the value in the `cdr` field.
    pub fn cdr(&self) -> &Value {
        &self.inner.1
    }

    /// Returns a mutable reference to the value in the `cdr` field.
    pub fn cdr_mut(&mut self) -> &mut Value {
        &mut self.inner.1
    }

    /// Sets the `cdr` field.
    pub fn set_cdr(&mut self, cdr: impl Into<Value>) {
        self.inner.1 = cdr.into()
    }

    /// Returns references to the values in the `car` and `cdr` fields.
    ///
    /// ```
    /// # use lexpr::{Cons, Value};
    /// let cell = Cons::new(1, 2);
    /// assert_eq!(cell.as_pair(), (&Value::from(1), &Value::from(2)));
    /// ```
    pub fn as_pair(&self) -> (&Value, &Value) {
        (&self.inner.0, &self.inner.1)
    }

    /// Converts `self` into a pair of values without cloning.
    ///
    /// ```
    /// # use lexpr::Cons;
    /// let cell = Cons::new("a", 42);
    /// assert_eq!(cell.car(), "a");
    /// assert_eq!(cell.cdr(), 42);
    /// let (car, cdr) = cell.into_pair();
    /// assert_eq!(car, "a");
    /// assert_eq!(cdr, 42);
    /// ```
    pub fn into_pair(self) -> (Value, Value) {
        (self.inner.0, self.inner.1)
    }

    /// Obtains an iterator yielding references to all the cons cells in this
    /// linked list.
    ///
    /// ```
    /// # use lexpr::{Cons, Value};
    /// for cell in Cons::new(1, Cons::new(2, Value::Null)).iter() {
    ///    println!("list element: {}", cell.car());
    /// }
    /// ```
    pub fn iter(&self) -> Iter<'_> {
        Iter { cursor: Some(self) }
    }

    /// Converts `self` into a vector without cloning the elements.
    ///
    /// Returns the accumulated items of the list and the `cdr` of the last list
    /// element. For proper lists, this will always be `Value::Null`.
    ///
    /// ```
    /// # use lexpr::{Cons, Value};
    /// let list = Cons::new(1, Cons::new(2, Cons::new(3, Value::Null)));
    /// assert_eq!(list.into_vec(), (vec![Value::from(1), Value::from(2), Value::from(3)], Value::Null));
    /// ```
    pub fn into_vec(self) -> (Vec<Value>, Value) {
        let mut vec = Vec::new();
        for (item, rest) in self.into_iter() {
            vec.push(item);
            if let Some(rest) = rest {
                return (vec, rest);
            }
        }
        unreachable!()
    }

    /// Retrieves a vector, cloning the values.
    ///
    /// Returns the accumulated items of the list and the `cdr` of the last list
    /// element. For proper lists, this will always be `Value::Null`.
    ///
    /// ```
    /// # use lexpr::{Cons, Value};
    /// let list = Cons::new(1, Cons::new(2, Cons::new(3, Value::Null)));
    /// assert_eq!(list.to_vec(), (vec![Value::from(1), Value::from(2), Value::from(3)], Value::Null));
    /// ```
    pub fn to_vec(&self) -> (Vec<Value>, Value) {
        let mut vec = Vec::new();
        for pair in self.iter() {
            vec.push(pair.car().clone());
            if !pair.cdr().is_cons() {
                return (vec, pair.cdr().clone());
            }
        }
        unreachable!()
    }

    /// Retrieves a vector, taking references to the values.
    ///
    /// Returns the accumulated items of the list and the `cdr` of the last list
    /// element. For proper lists, this will always be `Value::Null`.
    ///
    /// ```
    /// # use lexpr::{Cons, Value};
    /// let list = Cons::new(1, Cons::new(2, Cons::new(3, Value::Null)));
    /// assert_eq!(list.to_ref_vec(), (vec![&Value::from(1), &Value::from(2), &Value::from(3)], &Value::Null));
    /// ```
    pub fn to_ref_vec(&self) -> (Vec<&Value>, &Value) {
        let mut vec = Vec::new();
        for pair in self.iter() {
            vec.push(pair.car());
            if !pair.cdr().is_cons() {
                return (vec, pair.cdr());
            }
        }
        unreachable!()
    }

    /// Returns an iterator that returns each element (`car` field) of a singly-linked list.
    ///
    /// The iterator returns `None` if a terminating value is encountered. For a
    /// dotted list, the iterator is not yet exhausted at that point, and
    /// produces the non-`Null` terminating value next.
    pub fn list_iter(&self) -> ListIter<'_> {
        ListIter::cons(self)
    }
}

impl IntoIterator for Cons {
    type Item = (Value, Option<Value>);
    type IntoIter = IntoIter;

    /// Obtains an iterator yielding the contents of the elements of this linked
    /// list.
    ///
    /// The returned iterator transfers ownership of the values contained in the
    /// list to the consumer of the iterator. For each cons cell but the last,
    /// the iterator yields a pair containing the value in the cell's `car`
    /// field and `None`. For the last cell, the yielded pair will contain the
    /// value of `car` and `Some(cdr)`.
    //
    /// ```
    /// # use lexpr::{Cons, Value};
    /// let vec: Vec<_> = Cons::new(1, Cons::new(2, 3)).into_iter().collect();
    /// assert_eq!(vec, vec![(Value::from(1), None), (Value::from(2), Some(Value::from(3)))]);
    /// ```
    fn into_iter(self) -> IntoIter {
        IntoIter { cursor: Some(self) }
    }
}

impl<'a> IntoIterator for &'a Cons {
    type Item = &'a Cons;
    type IntoIter = Iter<'a>;

    fn into_iter(self) -> Iter<'a> {
        self.iter()
    }
}

/// An iterator over a chain of cons cells.
///
/// This is returned by the [`Cons::iter`] method.
pub struct Iter<'a> {
    cursor: Option<&'a Cons>,
}

impl<'a> Iter<'a> {
    /// Returns the current cons cell, without advancing the iterator.
    pub fn peek(&self) -> Option<&Cons> {
        self.cursor
    }
}

impl<'a> Iterator for Iter<'a> {
    type Item = &'a Cons;

    fn next(&mut self) -> Option<Self::Item> {
        match self.cursor {
            Some(pair) => {
                match pair.cdr() {
                    Value::Cons(pair) => self.cursor = Some(pair),
                    _ => self.cursor = None,
                }
                Some(pair)
            }
            None => None,
        }
    }
}

/// An iterator consuming a chain of cons cells.
///
/// This is returned by the [`Cons::into_iter`] method.
///
/// [`Cons::into_iter`]: struct.Cons.html#method.into_iter
pub struct IntoIter {
    cursor: Option<Cons>,
}

impl IntoIter {
    /// Returns the current cons cell, without advancing the iterator.
    pub fn peek(&self) -> Option<&Cons> {
        self.cursor.as_ref()
    }

    /// Returns a mutable reference to the current cons cell, without advancing
    /// the iterator.
    pub fn peek_mut(&mut self) -> Option<&mut Cons> {
        self.cursor.as_mut()
    }
}

impl Iterator for IntoIter {
    type Item = (Value, Option<Value>);

    fn next(&mut self) -> Option<Self::Item> {
        match self.cursor.take() {
            Some(cell) => {
                let (car, cdr) = cell.into_pair();
                match cdr {
                    Value::Cons(cell) => {
                        self.cursor = Some(cell);
                        Some((car, None))
                    }
                    _ => {
                        self.cursor = None;
                        Some((car, Some(cdr)))
                    }
                }
            }
            None => None,
        }
    }
}

/// An iterator yielding the `car` field of a chain of cons cells.
///
/// # Improper lists
///
/// Since in Lisp, lists can be "improper", i.e., terminated by a value other than `Null`, this
/// iterator type takes advantage of the fact that Rust's iterators can produce multiple sequences
/// of values, each terminated by `None`. For an improper list, the terminating value is produced
/// after the sequence of elements, as a singleton element, again followed by `None`.
///
/// For example, while the list `(1 2 3)` will produce the three expected `Some` values, followed by
/// `None`, the list `(1 2 . 3)` will produce `Some` values for `1` and `2`, then a `None`, followed
/// by a some value for `3`, and then the final `None`.
#[derive(Debug, Clone)]
pub struct ListIter<'a>(ListCursor<'a>);

#[derive(Debug, Clone)]
enum ListCursor<'a> {
    Cons(&'a Cons),
    Dot(&'a Value),
    Rest(&'a Value),
    Exhausted,
}

impl<'a> ListIter<'a> {
    /// Returns true when the iterator is completely exhausted.
    ///
    /// For an improper list, true will only be returned after the terminating value has been
    /// consumed.
    pub fn is_empty(&self) -> bool {
        match &self.0 {
            ListCursor::Exhausted => true,
            _ => false,
        }
    }

    /// Returns a peek at the value that would be returned by a call to `next`.
    ///
    /// For improper lists, this implies that after the last regular element, `None` will be
    /// returned, while `is_empty` still returns false at that point.
    pub fn peek(&self) -> Option<&Value> {
        match &self.0 {
            ListCursor::Cons(cell) => Some(cell.car()),
            ListCursor::Dot(_) => None,
            ListCursor::Rest(value) => Some(value),
            ListCursor::Exhausted => None,
        }
    }

    pub(crate) fn empty() -> Self {
        ListIter(ListCursor::Exhausted)
    }

    pub(crate) fn cons(cell: &'a Cons) -> Self {
        ListIter(ListCursor::Cons(cell))
    }
}

impl<'a> Iterator for ListIter<'a> {
    type Item = &'a Value;

    fn next(&mut self) -> Option<Self::Item> {
        match self.0 {
            ListCursor::Cons(cell) => {
                let car = cell.car();
                match cell.cdr() {
                    Value::Cons(next) => {
                        self.0 = ListCursor::Cons(next);
                    }
                    Value::Null => {
                        self.0 = ListCursor::Exhausted;
                    }
                    cdr => {
                        self.0 = ListCursor::Dot(cdr);
                    }
                }
                Some(car)
            }
            ListCursor::Dot(value) => {
                self.0 = ListCursor::Rest(value);
                None
            }
            ListCursor::Rest(value) => {
                self.0 = ListCursor::Exhausted;
                Some(value)
            }
            ListCursor::Exhausted => None,
        }
    }
}