about summary refs log tree commit diff
path: root/tvix/eval/src/value/thunk.rs
blob: 0290e73ebb7d29fed522e681274e647db3dcec9a (plain) (blame)
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
//! This module implements the runtime representation of Thunks.
//!
//! Thunks are a special kind of Nix value, similar to a 0-argument
//! closure that yields some value. Thunks are used to implement the
//! lazy evaluation behaviour of Nix:
//!
//! Whenever the compiler determines that an expression should be
//! evaluated lazily, it creates a thunk instead of compiling the
//! expression value directly. At any point in the runtime where the
//! actual value of a thunk is required, it is "forced", meaning that
//! the encompassing computation takes place and the thunk takes on
//! its new value.
//!
//! Thunks have interior mutability to be able to memoise their
//! computation. Once a thunk is evaluated, its internal
//! representation becomes the result of the expression. It is legal
//! for the runtime to replace a thunk object directly with its value
//! object, but when forcing a thunk, the runtime *must* mutate the
//! memoisable slot.

use std::{
    cell::{Ref, RefCell, RefMut},
    collections::HashSet,
    fmt::Debug,
    rc::Rc,
};

use crate::{
    errors::ErrorKind,
    spans::LightSpan,
    upvalues::Upvalues,
    value::Closure,
    vm::generators::{self, GenCo},
    Value,
};

use super::{Lambda, TotalDisplay};

/// Internal representation of a suspended native thunk.
struct SuspendedNative(Box<dyn Fn() -> Result<Value, ErrorKind>>);

impl Debug for SuspendedNative {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "SuspendedNative({:p})", self.0)
    }
}

/// Internal representation of the different states of a thunk.
///
/// Upvalues must be finalised before leaving the initial state
/// (Suspended or RecursiveClosure).  The [`value()`] function may
/// not be called until the thunk is in the final state (Evaluated).
#[derive(Debug)]
enum ThunkRepr {
    /// Thunk is closed over some values, suspended and awaiting
    /// execution.
    Suspended {
        lambda: Rc<Lambda>,
        upvalues: Rc<Upvalues>,
        light_span: LightSpan,
    },

    /// Thunk is a suspended native computation.
    Native(SuspendedNative),

    /// Thunk currently under-evaluation; encountering a blackhole
    /// value means that infinite recursion has occured.
    Blackhole { forced_at: LightSpan },

    /// Fully evaluated thunk.
    Evaluated(Value),
}

impl ThunkRepr {
    fn debug_repr(&self) -> String {
        match self {
            ThunkRepr::Evaluated(v) => format!("thunk(val|{})", v),
            ThunkRepr::Blackhole { .. } => "thunk(blackhole)".to_string(),
            ThunkRepr::Native(_) => "thunk(native)".to_string(),
            ThunkRepr::Suspended { lambda, .. } => format!("thunk({:p})", *lambda),
        }
    }
}

/// A thunk is created for any value which requires non-strict
/// evaluation due to self-reference or lazy semantics (or both).
/// Every reference cycle involving `Value`s will contain at least
/// one `Thunk`.
#[derive(Clone, Debug)]
pub struct Thunk(Rc<RefCell<ThunkRepr>>);

impl Thunk {
    pub fn new_closure(lambda: Rc<Lambda>) -> Self {
        Thunk(Rc::new(RefCell::new(ThunkRepr::Evaluated(Value::Closure(
            Rc::new(Closure {
                upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
                lambda: lambda.clone(),
            }),
        )))))
    }

    pub fn new_suspended(lambda: Rc<Lambda>, light_span: LightSpan) -> Self {
        Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
            upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
            lambda: lambda.clone(),
            light_span,
        })))
    }

    pub fn new_suspended_native(native: Box<dyn Fn() -> Result<Value, ErrorKind>>) -> Self {
        Thunk(Rc::new(RefCell::new(ThunkRepr::Native(SuspendedNative(
            native,
        )))))
    }

    // TODO(amjoseph): de-asyncify this
    pub async fn force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
        // If the current thunk is already fully evaluated, return its evaluated
        // value. The VM will continue running the code that landed us here.
        if self.is_forced() {
            return Ok(self.value().clone());
        }

        // Begin evaluation of this thunk by marking it as a blackhole, meaning
        // that any other forcing frame encountering this thunk before its
        // evaluation is completed detected an evaluation cycle.
        let inner = self.0.replace(ThunkRepr::Blackhole { forced_at: span });

        match inner {
            // If there was already a blackhole in the thunk, this is an
            // evaluation cycle.
            ThunkRepr::Blackhole { forced_at } => Err(ErrorKind::InfiniteRecursion {
                first_force: forced_at.span(),
            }),

            // If there is a native function stored in the thunk, evaluate it
            // and replace this thunk's representation with the result.
            ThunkRepr::Native(native) => {
                let value = native.0()?;

                // Force the returned value again, in case the native call
                // returned a thunk.
                let value = generators::request_force(&co, value).await;

                self.0.replace(ThunkRepr::Evaluated(value.clone()));
                Ok(value)
            }

            // When encountering a suspended thunk, request that the VM enters
            // it and produces the result.
            //
            ThunkRepr::Suspended {
                lambda,
                upvalues,
                light_span,
            } => {
                let value =
                    generators::request_enter_lambda(&co, lambda, upvalues, light_span).await;

                // This may have returned another thunk, so we need to request
                // that the VM forces this value, too.
                let value = generators::request_force(&co, value).await;

                self.0.replace(ThunkRepr::Evaluated(value.clone()));
                Ok(value)
            }

            // If an inner value is found, force it and then update. This is
            // most likely an inner thunk, as `Thunk:is_forced` returned false.
            ThunkRepr::Evaluated(val) => {
                let value = generators::request_force(&co, val).await;
                self.0.replace(ThunkRepr::Evaluated(value.clone()));
                Ok(value)
            }
        }
    }

    pub fn finalise(&self, stack: &[Value]) {
        self.upvalues_mut().resolve_deferred_upvalues(stack);
    }

    pub fn is_evaluated(&self) -> bool {
        matches!(*self.0.borrow(), ThunkRepr::Evaluated(_))
    }

    pub fn is_suspended(&self) -> bool {
        matches!(
            *self.0.borrow(),
            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_)
        )
    }

    /// Returns true if forcing this thunk will not change it.
    pub fn is_forced(&self) -> bool {
        match *self.0.borrow() {
            ThunkRepr::Evaluated(Value::Thunk(_)) => false,
            ThunkRepr::Evaluated(_) => true,
            _ => false,
        }
    }

    /// Returns a reference to the inner evaluated value of a thunk.
    /// It is an error to call this on a thunk that has not been
    /// forced, or is not otherwise known to be fully evaluated.
    // Note: Due to the interior mutability of thunks this is
    // difficult to represent in the type system without impacting the
    // API too much.
    pub fn value(&self) -> Ref<Value> {
        Ref::map(self.0.borrow(), |thunk| match thunk {
            ThunkRepr::Evaluated(value) => value,
            ThunkRepr::Blackhole { .. } => panic!("Thunk::value called on a black-holed thunk"),
            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => {
                panic!("Thunk::value called on a suspended thunk")
            }
        })
    }

    pub fn upvalues(&self) -> Ref<'_, Upvalues> {
        Ref::map(self.0.borrow(), |thunk| match thunk {
            ThunkRepr::Suspended { upvalues, .. } => upvalues.as_ref(),
            ThunkRepr::Evaluated(Value::Closure(c)) => &c.upvalues,
            _ => panic!("upvalues() on non-suspended thunk"),
        })
    }

    pub fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
        RefMut::map(self.0.borrow_mut(), |thunk| match thunk {
            ThunkRepr::Suspended { upvalues, .. } => Rc::get_mut(upvalues).unwrap(),
            ThunkRepr::Evaluated(Value::Closure(c)) => Rc::get_mut(
                &mut Rc::get_mut(c).unwrap().upvalues,
            )
            .expect(
                "upvalues_mut() was called on a thunk which already had multiple references to it",
            ),
            thunk => panic!("upvalues() on non-suspended thunk: {thunk:?}"),
        })
    }

    /// Do not use this without first reading and understanding
    /// `tvix/docs/value-pointer-equality.md`.
    pub(crate) fn ptr_eq(&self, other: &Self) -> bool {
        if Rc::ptr_eq(&self.0, &other.0) {
            return true;
        }
        match &*self.0.borrow() {
            ThunkRepr::Evaluated(Value::Closure(c1)) => match &*other.0.borrow() {
                ThunkRepr::Evaluated(Value::Closure(c2)) => Rc::ptr_eq(c1, c2),
                _ => false,
            },
            _ => false,
        }
    }

    /// Helper function to format thunks in observer output.
    pub(crate) fn debug_repr(&self) -> String {
        self.0.borrow().debug_repr()
    }
}

impl TotalDisplay for Thunk {
    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
        if !set.insert(self) {
            return f.write_str("<CYCLE>");
        }

        match &*self.0.borrow() {
            ThunkRepr::Evaluated(v) => v.total_fmt(f, set),
            other => write!(f, "internal[{}]", other.debug_repr()),
        }
    }
}

/// A wrapper type for tracking which thunks have already been seen in a
/// context. This is necessary for cycle detection.
///
/// The inner `HashSet` is not available on the outside, as it would be
/// potentially unsafe to interact with the pointers in the set.
#[derive(Default)]
pub struct ThunkSet(HashSet<*const ThunkRepr>);

impl ThunkSet {
    /// Check whether the given thunk has already been seen. Will mark the thunk
    /// as seen otherwise.
    pub fn insert(&mut self, thunk: &Thunk) -> bool {
        let ptr: *const ThunkRepr = thunk.0.as_ptr();
        self.0.insert(ptr)
    }
}

#[derive(Default, Clone)]
pub struct SharedThunkSet(Rc<RefCell<ThunkSet>>);

impl SharedThunkSet {
    /// Check whether the given thunk has already been seen. Will mark the thunk
    /// as seen otherwise.
    pub fn insert(&self, thunk: &Thunk) -> bool {
        self.0.borrow_mut().insert(thunk)
    }
}