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
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
//! This module implements the backing representation of runtime
//! values in the Nix language.
use std::cmp::Ordering;
use std::fmt::Display;
use std::num::{NonZeroI32, NonZeroUsize};
use std::path::PathBuf;
use std::rc::Rc;

use bstr::{BString, ByteVec};
use codemap::Span;
use lexical_core::format::CXX_LITERAL;
use serde::Deserialize;

#[cfg(feature = "arbitrary")]
mod arbitrary;
mod attrs;
mod builtin;
mod function;
mod json;
mod list;
mod path;
mod string;
mod thunk;

use crate::errors::{CatchableErrorKind, ErrorKind};
use crate::opcode::StackIdx;
use crate::vm::generators::{self, GenCo};
use crate::AddContext;
pub use attrs::NixAttrs;
pub use builtin::{Builtin, BuiltinResult};
pub(crate) use function::Formals;
pub use function::{Closure, Lambda};
pub use list::NixList;
pub use path::canon_path;
pub use string::{NixContext, NixContextElement, NixString};
pub use thunk::Thunk;

pub use self::thunk::ThunkSet;

use lazy_static::lazy_static;

#[warn(variant_size_differences)]
#[derive(Clone, Debug, Deserialize)]
#[serde(untagged)]
pub enum Value {
    Null,
    Bool(bool),
    Integer(i64),
    Float(f64),
    String(NixString),

    #[serde(skip)]
    Path(Box<PathBuf>),
    Attrs(Box<NixAttrs>),
    List(NixList),

    #[serde(skip)]
    Closure(Rc<Closure>), // must use Rc<Closure> here in order to get proper pointer equality

    #[serde(skip)]
    Builtin(Builtin),

    // Internal values that, while they technically exist at runtime,
    // are never returned to or created directly by users.
    #[serde(skip_deserializing)]
    Thunk(Thunk),

    // See [`compiler::compile_select_or()`] for explanation
    #[serde(skip)]
    AttrNotFound,

    // this can only occur in Chunk::Constants and nowhere else
    #[serde(skip)]
    Blueprint(Rc<Lambda>),

    #[serde(skip)]
    DeferredUpvalue(StackIdx),
    #[serde(skip)]
    UnresolvedPath(Box<PathBuf>),
    #[serde(skip)]
    Json(Box<(serde_json::Value, NixContext)>),

    #[serde(skip)]
    FinaliseRequest(bool),

    #[serde(skip)]
    Catchable(Box<CatchableErrorKind>),
}

impl From<CatchableErrorKind> for Value {
    #[inline]
    fn from(c: CatchableErrorKind) -> Value {
        Value::Catchable(Box::new(c))
    }
}

impl<V> From<Result<V, CatchableErrorKind>> for Value
where
    Value: From<V>,
{
    #[inline]
    fn from(v: Result<V, CatchableErrorKind>) -> Value {
        match v {
            Ok(v) => v.into(),
            Err(e) => Value::Catchable(Box::new(e)),
        }
    }
}

lazy_static! {
    static ref WRITE_FLOAT_OPTIONS: lexical_core::WriteFloatOptions =
        lexical_core::WriteFloatOptionsBuilder::new()
            .trim_floats(true)
            .round_mode(lexical_core::write_float_options::RoundMode::Round)
            .positive_exponent_break(Some(NonZeroI32::new(5).unwrap()))
            .max_significant_digits(Some(NonZeroUsize::new(6).unwrap()))
            .build()
            .unwrap();
}

// Helper macros to generate the to_*/as_* macros while accounting for
// thunks.

/// Generate an `as_*` method returning a reference to the expected
/// type, or a type error. This only works for types that implement
/// `Copy`, as returning a reference to an inner thunk value is not
/// possible.

/// Generate an `as_*/to_*` accessor method that returns either the
/// expected type, or a type error.
macro_rules! gen_cast {
    ( $name:ident, $type:ty, $expected:expr, $variant:pat, $result:expr ) => {
        pub fn $name(&self) -> Result<$type, ErrorKind> {
            match self {
                $variant => Ok($result),
                Value::Thunk(thunk) => Self::$name(&thunk.value()),
                other => Err(type_error($expected, &other)),
            }
        }
    };
}

/// Generate an `as_*_mut/to_*_mut` accessor method that returns either the
/// expected type, or a type error.
macro_rules! gen_cast_mut {
    ( $name:ident, $type:ty, $expected:expr, $variant:ident) => {
        pub fn $name(&mut self) -> Result<&mut $type, ErrorKind> {
            match self {
                Value::$variant(x) => Ok(x),
                other => Err(type_error($expected, &other)),
            }
        }
    };
}

/// Generate an `is_*` type-checking method.
macro_rules! gen_is {
    ( $name:ident, $variant:pat ) => {
        pub fn $name(&self) -> bool {
            match self {
                $variant => true,
                Value::Thunk(thunk) => Self::$name(&thunk.value()),
                _ => false,
            }
        }
    };
}

/// Describes what input types are allowed when coercing a `Value` to a string
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct CoercionKind {
    /// If false only coerce already "stringly" types like strings and paths, but
    /// also coerce sets that have a `__toString` attribute. In Tvix, this is
    /// usually called a weak coercion. Equivalent to passing `false` as the
    /// `coerceMore` argument of `EvalState::coerceToString` in C++ Nix.
    ///
    /// If true coerce all value types included by a weak coercion, but also
    /// coerce `null`, booleans, integers, floats and lists of coercible types.
    /// Consequently, we call this a strong coercion. Equivalent to passing
    /// `true` as `coerceMore` in C++ Nix.
    pub strong: bool,

    /// If `import_paths` is `true`, paths are imported into the store and their
    /// store path is the result of the coercion (equivalent to the
    /// `copyToStore` argument of `EvalState::coerceToString` in C++ Nix).
    pub import_paths: bool,
}

impl From<CoercionKind> for u8 {
    fn from(k: CoercionKind) -> u8 {
        k.strong as u8 | (k.import_paths as u8) << 1
    }
}

impl From<u8> for CoercionKind {
    fn from(byte: u8) -> Self {
        CoercionKind {
            strong: byte & 0x01 != 0,
            import_paths: byte & 0x02 != 0,
        }
    }
}

impl<T> From<T> for Value
where
    T: Into<NixString>,
{
    fn from(t: T) -> Self {
        Self::String(t.into())
    }
}

/// Controls what kind of by-pointer equality comparison is allowed.
///
/// See `//tvix/docs/value-pointer-equality.md` for details.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum PointerEquality {
    /// Pointer equality not allowed at all.
    ForbidAll,

    /// Pointer equality comparisons only allowed for nested values.
    AllowNested,

    /// Pointer equality comparisons are allowed in all contexts.
    AllowAll,
}

impl Value {
    /// Construct a [`Value::Attrs`] from a [`NixAttrs`].
    pub fn attrs(attrs: NixAttrs) -> Self {
        Self::Attrs(Box::new(attrs))
    }

    /// Deeply forces a value, traversing e.g. lists and attribute sets and forcing
    /// their contents, too.
    ///
    /// This is a generator function.
    pub(super) async fn deep_force(self, co: GenCo, span: Span) -> Result<Value, ErrorKind> {
        if let Some(v) = Self::deep_force_(self.clone(), co, span).await? {
            Ok(v)
        } else {
            Ok(self)
        }
    }

    /// Returns Some(v) or None to indicate the returned value is myself
    async fn deep_force_(myself: Value, co: GenCo, span: Span) -> Result<Option<Value>, ErrorKind> {
        // This is a stack of values which still remain to be forced.
        let mut vals = vec![myself];

        let mut thunk_set: ThunkSet = Default::default();

        loop {
            let v = if let Some(v) = vals.pop() {
                v
            } else {
                return Ok(None);
            };

            // Get rid of any top-level thunks, and bail out of self-recursive
            // thunks.
            let value = if let Value::Thunk(t) = &v {
                if !thunk_set.insert(t) {
                    continue;
                }
                Thunk::force_(t.clone(), &co, span).await?
            } else {
                v
            };

            match value {
                // Short-circuit on already evaluated values, or fail on internal values.
                Value::Null
                | Value::Bool(_)
                | Value::Integer(_)
                | Value::Float(_)
                | Value::String(_)
                | Value::Path(_)
                | Value::Closure(_)
                | Value::Builtin(_) => continue,

                Value::List(list) => {
                    for val in list.into_iter().rev() {
                        vals.push(val);
                    }
                    continue;
                }

                Value::Attrs(attrs) => {
                    for (_, val) in attrs.into_iter().rev() {
                        vals.push(val);
                    }
                    continue;
                }

                Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),

                Value::Catchable(_) => return Ok(Some(value)),

                Value::AttrNotFound
                | Value::Blueprint(_)
                | Value::DeferredUpvalue(_)
                | Value::UnresolvedPath(_)
                | Value::Json(..)
                | Value::FinaliseRequest(_) => panic!(
                    "Tvix bug: internal value left on stack: {}",
                    value.type_of()
                ),
            }
        }
    }

    pub async fn coerce_to_string(
        self,
        co: GenCo,
        kind: CoercionKind,
        span: Span,
    ) -> Result<Value, ErrorKind> {
        self.coerce_to_string_(&co, kind, span).await
    }

    /// Coerce a `Value` to a string. See `CoercionKind` for a rundown of what
    /// input types are accepted under what circumstances.
    pub async fn coerce_to_string_(
        self,
        co: &GenCo,
        kind: CoercionKind,
        span: Span,
    ) -> Result<Value, ErrorKind> {
        let mut result = BString::default();
        let mut vals = vec![self];
        // Track if we are coercing the first value of a list to correctly emit
        // separating white spaces.
        let mut is_list_head = None;
        // FIXME(raitobezarius): as per https://b.tvl.fyi/issues/364
        // we might be interested into more powerful context-related coercion kinds.
        let mut context: NixContext = NixContext::new();

        loop {
            let value = if let Some(v) = vals.pop() {
                v.force(co, span).await?
            } else {
                return Ok(Value::String(NixString::new_context_from(context, result)));
            };
            let coerced: Result<BString, _> = match (value, kind) {
                // coercions that are always done
                (Value::String(mut s), _) => {
                    if let Some(ctx) = s.take_context() {
                        context.extend(ctx.into_iter());
                    }
                    Ok((*s).into())
                }

                // TODO(sterni): Think about proper encoding handling here. This needs
                // general consideration anyways, since one current discrepancy between
                // C++ Nix and Tvix is that the former's strings are arbitrary byte
                // sequences without NUL bytes, whereas Tvix only allows valid
                // Unicode. See also b/189.
                (
                    Value::Path(p),
                    CoercionKind {
                        import_paths: true, ..
                    },
                ) => {
                    let imported = generators::request_path_import(co, *p).await;
                    // When we import a path from the evaluator, we must attach
                    // its original path as its context.
                    context = context.append(NixContextElement::Plain(
                        imported.to_string_lossy().to_string(),
                    ));
                    Ok(imported.into_os_string().into_encoded_bytes().into())
                }
                (
                    Value::Path(p),
                    CoercionKind {
                        import_paths: false,
                        ..
                    },
                ) => Ok(p.into_os_string().into_encoded_bytes().into()),

                // Attribute sets can be converted to strings if they either have an
                // `__toString` attribute which holds a function that receives the
                // set itself or an `outPath` attribute which should be a string.
                // `__toString` is preferred.
                (Value::Attrs(attrs), kind) => {
                    if let Some(to_string) = attrs.select("__toString") {
                        let callable = to_string.clone().force(co, span).await?;

                        // Leave the attribute set on the stack as an argument
                        // to the function call.
                        generators::request_stack_push(co, Value::Attrs(attrs.clone())).await;

                        // Call the callable ...
                        let result = generators::request_call(co, callable).await;

                        // Recurse on the result, as attribute set coercion
                        // actually works recursively, e.g. you can even return
                        // /another/ set with a __toString attr.
                        vals.push(result);
                        continue;
                    } else if let Some(out_path) = attrs.select("outPath") {
                        vals.push(out_path.clone());
                        continue;
                    } else {
                        return Err(ErrorKind::NotCoercibleToString { from: "set", kind });
                    }
                }

                // strong coercions
                (Value::Null, CoercionKind { strong: true, .. })
                | (Value::Bool(false), CoercionKind { strong: true, .. }) => Ok("".into()),
                (Value::Bool(true), CoercionKind { strong: true, .. }) => Ok("1".into()),

                (Value::Integer(i), CoercionKind { strong: true, .. }) => Ok(format!("{i}").into()),
                (Value::Float(f), CoercionKind { strong: true, .. }) => {
                    // contrary to normal Display, coercing a float to a string will
                    // result in unconditional 6 decimal places
                    Ok(format!("{:.6}", f).into())
                }

                // Lists are coerced by coercing their elements and interspersing spaces
                (Value::List(list), CoercionKind { strong: true, .. }) => {
                    for elem in list.into_iter().rev() {
                        vals.push(elem);
                    }
                    // In case we are coercing a list within a list we don't want
                    // to touch this. Since the algorithm is nonrecursive, the
                    // space would not have been created yet (due to continue).
                    if is_list_head.is_none() {
                        is_list_head = Some(true);
                    }
                    continue;
                }

                (Value::Thunk(_), _) => panic!("Tvix bug: force returned unforced thunk"),

                val @ (Value::Closure(_), _)
                | val @ (Value::Builtin(_), _)
                | val @ (Value::Null, _)
                | val @ (Value::Bool(_), _)
                | val @ (Value::Integer(_), _)
                | val @ (Value::Float(_), _)
                | val @ (Value::List(_), _) => Err(ErrorKind::NotCoercibleToString {
                    from: val.0.type_of(),
                    kind,
                }),

                (c @ Value::Catchable(_), _) => return Ok(c),

                (Value::AttrNotFound, _)
                | (Value::Blueprint(_), _)
                | (Value::DeferredUpvalue(_), _)
                | (Value::UnresolvedPath(_), _)
                | (Value::Json(..), _)
                | (Value::FinaliseRequest(_), _) => {
                    panic!("tvix bug: .coerce_to_string() called on internal value")
                }
            };

            if let Some(head) = is_list_head {
                if !head {
                    result.push(b' ');
                } else {
                    is_list_head = Some(false);
                }
            }

            result.push_str(&coerced?);
        }
    }

    pub(crate) async fn nix_eq_owned_genco(
        self,
        other: Value,
        co: GenCo,
        ptr_eq: PointerEquality,
        span: Span,
    ) -> Result<Value, ErrorKind> {
        self.nix_eq(other, &co, ptr_eq, span).await
    }

    /// Compare two Nix values for equality, forcing nested parts of the structure
    /// as needed.
    ///
    /// This comparison needs to be invoked for nested values (e.g. in lists and
    /// attribute sets) as well, which is done by suspending and asking the VM to
    /// perform the nested comparison.
    ///
    /// The `top_level` parameter controls whether this invocation is the top-level
    /// comparison, or a nested value comparison. See
    /// `//tvix/docs/value-pointer-equality.md`
    pub(crate) async fn nix_eq(
        self,
        other: Value,
        co: &GenCo,
        ptr_eq: PointerEquality,
        span: Span,
    ) -> Result<Value, ErrorKind> {
        // this is a stack of ((v1,v2),peq) triples to be compared;
        // after each triple is popped off of the stack, v1 is
        // compared to v2 using peq-mode PointerEquality
        let mut vals = vec![((self, other), ptr_eq)];

        loop {
            let ((a, b), ptr_eq) = if let Some(abp) = vals.pop() {
                abp
            } else {
                // stack is empty, so comparison has succeeded
                return Ok(Value::Bool(true));
            };
            let a = match a {
                Value::Thunk(thunk) => {
                    // If both values are thunks, and thunk comparisons are allowed by
                    // pointer, do that and move on.
                    if ptr_eq == PointerEquality::AllowAll {
                        if let Value::Thunk(t1) = &b {
                            if t1.ptr_eq(&thunk) {
                                continue;
                            }
                        }
                    };

                    Thunk::force_(thunk, co, span).await?
                }

                _ => a,
            };

            let b = b.force(co, span).await?;

            debug_assert!(!matches!(a, Value::Thunk(_)));
            debug_assert!(!matches!(b, Value::Thunk(_)));

            let result = match (a, b) {
                // Trivial comparisons
                (c @ Value::Catchable(_), _) => return Ok(c),
                (_, c @ Value::Catchable(_)) => return Ok(c),
                (Value::Null, Value::Null) => true,
                (Value::Bool(b1), Value::Bool(b2)) => b1 == b2,
                (Value::String(s1), Value::String(s2)) => s1 == s2,
                (Value::Path(p1), Value::Path(p2)) => p1 == p2,

                // Numerical comparisons (they work between float & int)
                (Value::Integer(i1), Value::Integer(i2)) => i1 == i2,
                (Value::Integer(i), Value::Float(f)) => i as f64 == f,
                (Value::Float(f1), Value::Float(f2)) => f1 == f2,
                (Value::Float(f), Value::Integer(i)) => i as f64 == f,

                // List comparisons
                (Value::List(l1), Value::List(l2)) => {
                    if ptr_eq >= PointerEquality::AllowNested && l1.ptr_eq(&l2) {
                        continue;
                    }

                    if l1.len() != l2.len() {
                        return Ok(Value::Bool(false));
                    }

                    vals.extend(l1.into_iter().rev().zip(l2.into_iter().rev()).zip(
                        std::iter::repeat(std::cmp::max(ptr_eq, PointerEquality::AllowNested)),
                    ));
                    continue;
                }

                (_, Value::List(_)) | (Value::List(_), _) => return Ok(Value::Bool(false)),

                // Attribute set comparisons
                (Value::Attrs(a1), Value::Attrs(a2)) => {
                    if ptr_eq >= PointerEquality::AllowNested && a1.ptr_eq(&a2) {
                        continue;
                    }

                    // Special-case for derivation comparisons: If both attribute sets
                    // have `type = derivation`, compare them by `outPath`.
                    #[allow(clippy::single_match)] // might need more match arms later
                    match (a1.select("type"), a2.select("type")) {
                        (Some(v1), Some(v2)) => {
                            let s1 = v1.clone().force(co, span).await?;
                            if s1.is_catchable() {
                                return Ok(s1);
                            }
                            let s2 = v2.clone().force(co, span).await?;
                            if s2.is_catchable() {
                                return Ok(s2);
                            }
                            let s1 = s1.to_str();
                            let s2 = s2.to_str();

                            if let (Ok(s1), Ok(s2)) = (s1, s2) {
                                if s1 == "derivation" && s2 == "derivation" {
                                    // TODO(tazjin): are the outPaths really required,
                                    // or should it fall through?
                                    let out1 = a1
                                        .select_required("outPath")
                                        .context("comparing derivations")?
                                        .clone();

                                    let out2 = a2
                                        .select_required("outPath")
                                        .context("comparing derivations")?
                                        .clone();

                                    let out1 = out1.clone().force(co, span).await?;
                                    let out2 = out2.clone().force(co, span).await?;

                                    if out1.is_catchable() {
                                        return Ok(out1);
                                    }

                                    if out2.is_catchable() {
                                        return Ok(out2);
                                    }

                                    let result =
                                        out1.to_contextful_str()? == out2.to_contextful_str()?;
                                    if !result {
                                        return Ok(Value::Bool(false));
                                    } else {
                                        continue;
                                    }
                                }
                            }
                        }
                        _ => {}
                    };

                    if a1.len() != a2.len() {
                        return Ok(Value::Bool(false));
                    }

                    // note that it is important to be careful here with the
                    // order we push the keys and values in order to properly
                    // compare attrsets containing `throw` elements.
                    let iter1 = a1.into_iter_sorted().rev();
                    let iter2 = a2.into_iter_sorted().rev();
                    for ((k1, v1), (k2, v2)) in iter1.zip(iter2) {
                        vals.push((
                            (v1, v2),
                            std::cmp::max(ptr_eq, PointerEquality::AllowNested),
                        ));
                        vals.push((
                            (k1.into(), k2.into()),
                            std::cmp::max(ptr_eq, PointerEquality::AllowNested),
                        ));
                    }
                    continue;
                }

                (Value::Attrs(_), _) | (_, Value::Attrs(_)) => return Ok(Value::Bool(false)),

                (Value::Closure(c1), Value::Closure(c2))
                    if ptr_eq >= PointerEquality::AllowNested =>
                {
                    if Rc::ptr_eq(&c1, &c2) {
                        continue;
                    } else {
                        return Ok(Value::Bool(false));
                    }
                }

                // Everything else is either incomparable (e.g. internal types) or
                // false.
                _ => return Ok(Value::Bool(false)),
            };
            if !result {
                return Ok(Value::Bool(false));
            }
        }
    }

    pub fn type_of(&self) -> &'static str {
        match self {
            Value::Null => "null",
            Value::Bool(_) => "bool",
            Value::Integer(_) => "int",
            Value::Float(_) => "float",
            Value::String(_) => "string",
            Value::Path(_) => "path",
            Value::Attrs(_) => "set",
            Value::List(_) => "list",
            Value::Closure(_) | Value::Builtin(_) => "lambda",

            // Internal types. Note: These are only elaborated here
            // because it makes debugging easier. If a user ever sees
            // any of these strings, it's a bug.
            Value::Thunk(_) => "internal[thunk]",
            Value::AttrNotFound => "internal[attr_not_found]",
            Value::Blueprint(_) => "internal[blueprint]",
            Value::DeferredUpvalue(_) => "internal[deferred_upvalue]",
            Value::UnresolvedPath(_) => "internal[unresolved_path]",
            Value::Json(..) => "internal[json]",
            Value::FinaliseRequest(_) => "internal[finaliser_sentinel]",
            Value::Catchable(_) => "internal[catchable]",
        }
    }

    gen_cast!(as_bool, bool, "bool", Value::Bool(b), *b);
    gen_cast!(as_int, i64, "int", Value::Integer(x), *x);
    gen_cast!(as_float, f64, "float", Value::Float(x), *x);

    /// Cast the current value into a **context-less** string.
    /// If you wanted to cast it into a potentially contextful string,
    /// you have to explicitly use `to_contextful_str`.
    /// Contextful strings are special, they should not be obtained
    /// everytime you want a string.
    pub fn to_str(&self) -> Result<NixString, ErrorKind> {
        match self {
            Value::String(s) if !s.has_context() => Ok((*s).clone()),
            Value::Thunk(thunk) => Self::to_str(&thunk.value()),
            other => Err(type_error("contextless strings", other)),
        }
    }

    gen_cast!(
        to_contextful_str,
        NixString,
        "contextful string",
        Value::String(s),
        (*s).clone()
    );
    gen_cast!(to_path, Box<PathBuf>, "path", Value::Path(p), p.clone());
    gen_cast!(to_attrs, Box<NixAttrs>, "set", Value::Attrs(a), a.clone());
    gen_cast!(to_list, NixList, "list", Value::List(l), l.clone());
    gen_cast!(
        as_closure,
        Rc<Closure>,
        "lambda",
        Value::Closure(c),
        c.clone()
    );

    gen_cast_mut!(as_list_mut, NixList, "list", List);

    gen_is!(is_path, Value::Path(_));
    gen_is!(is_number, Value::Integer(_) | Value::Float(_));
    gen_is!(is_bool, Value::Bool(_));
    gen_is!(is_attrs, Value::Attrs(_));
    gen_is!(is_catchable, Value::Catchable(_));

    /// Returns `true` if the value is a [`Thunk`].
    ///
    /// [`Thunk`]: Value::Thunk
    pub fn is_thunk(&self) -> bool {
        matches!(self, Self::Thunk(..))
    }

    /// Compare `self` against other using (fallible) Nix ordering semantics.
    ///
    /// The function is intended to be used from within other generator
    /// functions or `gen!` blocks.
    pub async fn nix_cmp_ordering(
        self,
        other: Self,
        co: GenCo,
        span: Span,
    ) -> Result<Result<Ordering, CatchableErrorKind>, ErrorKind> {
        Self::nix_cmp_ordering_(self, other, co, span).await
    }

    async fn nix_cmp_ordering_(
        myself: Self,
        other: Self,
        co: GenCo,
        span: Span,
    ) -> Result<Result<Ordering, CatchableErrorKind>, ErrorKind> {
        // this is a stack of ((v1,v2),peq) triples to be compared;
        // after each triple is popped off of the stack, v1 is
        // compared to v2 using peq-mode PointerEquality
        let mut vals = vec![((myself, other), PointerEquality::ForbidAll)];

        loop {
            let ((mut a, mut b), ptr_eq) = if let Some(abp) = vals.pop() {
                abp
            } else {
                // stack is empty, so they are equal
                return Ok(Ok(Ordering::Equal));
            };
            if ptr_eq == PointerEquality::AllowAll {
                if a.clone()
                    .nix_eq(b.clone(), &co, PointerEquality::AllowAll, span)
                    .await?
                    .as_bool()?
                {
                    continue;
                }
                a = a.force(&co, span).await?;
                b = b.force(&co, span).await?;
            }
            let result = match (a, b) {
                (Value::Catchable(c), _) => return Ok(Err(*c)),
                (_, Value::Catchable(c)) => return Ok(Err(*c)),
                // same types
                (Value::Integer(i1), Value::Integer(i2)) => i1.cmp(&i2),
                (Value::Float(f1), Value::Float(f2)) => f1.total_cmp(&f2),
                (Value::String(s1), Value::String(s2)) => s1.cmp(&s2),
                (Value::List(l1), Value::List(l2)) => {
                    let max = l1.len().max(l2.len());
                    for j in 0..max {
                        let i = max - 1 - j;
                        if i >= l2.len() {
                            vals.push(((1.into(), 0.into()), PointerEquality::ForbidAll));
                        } else if i >= l1.len() {
                            vals.push(((0.into(), 1.into()), PointerEquality::ForbidAll));
                        } else {
                            vals.push(((l1[i].clone(), l2[i].clone()), PointerEquality::AllowAll));
                        }
                    }
                    continue;
                }

                // different types
                (Value::Integer(i1), Value::Float(f2)) => (i1 as f64).total_cmp(&f2),
                (Value::Float(f1), Value::Integer(i2)) => f1.total_cmp(&(i2 as f64)),

                // unsupported types
                (lhs, rhs) => {
                    return Err(ErrorKind::Incomparable {
                        lhs: lhs.type_of(),
                        rhs: rhs.type_of(),
                    })
                }
            };
            if result != Ordering::Equal {
                return Ok(Ok(result));
            }
        }
    }

    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
    pub async fn force(self, co: &GenCo, span: Span) -> Result<Value, ErrorKind> {
        if let Value::Thunk(thunk) = self {
            // TODO(amjoseph): use #[tailcall::mutual]
            return Thunk::force_(thunk, co, span).await;
        }
        Ok(self)
    }

    // need two flavors, because async
    pub async fn force_owned_genco(self, co: GenCo, span: Span) -> Result<Value, ErrorKind> {
        if let Value::Thunk(thunk) = self {
            // TODO(amjoseph): use #[tailcall::mutual]
            return Thunk::force_(thunk, &co, span).await;
        }
        Ok(self)
    }

    /// Explain a value in a human-readable way, e.g. by presenting
    /// the docstrings of functions if present.
    pub fn explain(&self) -> String {
        match self {
            Value::Null => "the 'null' value".into(),
            Value::Bool(b) => format!("the boolean value '{}'", b),
            Value::Integer(i) => format!("the integer '{}'", i),
            Value::Float(f) => format!("the float '{}'", f),
            Value::String(s) if s.has_context() => format!("the contextful string '{}'", s),
            Value::String(s) => format!("the contextless string '{}'", s),
            Value::Path(p) => format!("the path '{}'", p.to_string_lossy()),
            Value::Attrs(attrs) => format!("a {}-item attribute set", attrs.len()),
            Value::List(list) => format!("a {}-item list", list.len()),

            Value::Closure(f) => {
                if let Some(name) = &f.lambda.name {
                    format!("the user-defined Nix function '{}'", name)
                } else {
                    "a user-defined Nix function".to_string()
                }
            }

            Value::Builtin(b) => {
                let mut out = format!("the builtin function '{}'", b.name());
                if let Some(docs) = b.documentation() {
                    out.push_str("\n\n");
                    out.push_str(docs);
                }
                out
            }

            // TODO: handle suspended thunks with a different explanation instead of panicking
            Value::Thunk(t) => t.value().explain(),

            Value::Catchable(_) => "a catchable failure".into(),

            Value::AttrNotFound
            | Value::Blueprint(_)
            | Value::DeferredUpvalue(_)
            | Value::UnresolvedPath(_)
            | Value::Json(..)
            | Value::FinaliseRequest(_) => "an internal Tvix evaluator value".into(),
        }
    }

    /// Constructs a thunk that will be evaluated lazily at runtime. This lets
    /// users of Tvix implement their own lazy builtins and so on.
    pub fn suspended_native_thunk(native: Box<dyn Fn() -> Result<Value, ErrorKind>>) -> Self {
        Value::Thunk(Thunk::new_suspended_native(native))
    }
}

trait TotalDisplay {
    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result;
}

impl Display for Value {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.total_fmt(f, &mut Default::default())
    }
}

/// Emulates the C++-Nix style formatting of floats, which diverges
/// significantly from Rust's native float formatting.
fn total_fmt_float<F: std::fmt::Write>(num: f64, mut f: F) -> std::fmt::Result {
    let mut buf = [b'0'; lexical_core::BUFFER_SIZE];
    let mut s = lexical_core::write_with_options::<f64, { CXX_LITERAL }>(
        num,
        &mut buf,
        &WRITE_FLOAT_OPTIONS,
    );

    // apply some postprocessing on the buffer. If scientific
    // notation is used (we see an `e`), and the next character is
    // a digit, add the missing `+` sign.)
    let mut new_s = Vec::with_capacity(s.len());

    if s.contains(&b'e') {
        for (i, c) in s.iter().enumerate() {
            // encountered `e`
            if c == &b'e' {
                // next character is a digit (so no negative exponent)
                if s.len() > i && s[i + 1].is_ascii_digit() {
                    // copy everything from the start up to (including) the e
                    new_s.extend_from_slice(&s[0..=i]);
                    // add the missing '+'
                    new_s.push(b'+');
                    // check for the remaining characters.
                    // If it's only one, we need to prepend a trailing zero
                    if s.len() == i + 2 {
                        new_s.push(b'0');
                    }
                    new_s.extend_from_slice(&s[i + 1..]);
                    break;
                }
            }
        }

        // if we modified the scientific notation, flip the reference
        if !new_s.is_empty() {
            s = &mut new_s
        }
    } else if s.contains(&b'.') {
        // else, if this is not scientific notation, and there's a
        // decimal point, make sure we really drop trailing zeroes.
        // In some cases, lexical_core doesn't.
        for (i, c) in s.iter().enumerate() {
            // at `.``
            if c == &b'.' {
                // trim zeroes from the right side.
                let frac = String::from_utf8_lossy(&s[i + 1..]);
                let frac_no_trailing_zeroes = frac.trim_end_matches('0');

                if frac.len() != frac_no_trailing_zeroes.len() {
                    // we managed to strip something, construct new_s
                    if frac_no_trailing_zeroes.is_empty() {
                        // if frac_no_trailing_zeroes is empty, the fractional part was all zeroes, so we can drop the decimal point as well
                        new_s.extend_from_slice(&s[0..=i - 1]);
                    } else {
                        // else, assemble the rest of the string
                        new_s.extend_from_slice(&s[0..=i]);
                        new_s.extend_from_slice(frac_no_trailing_zeroes.as_bytes());
                    }

                    // flip the reference
                    s = &mut new_s;
                    break;
                }
            }
        }
    }

    write!(f, "{}", String::from_utf8_lossy(s))
}

impl TotalDisplay for Value {
    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
        match self {
            Value::Null => f.write_str("null"),
            Value::Bool(true) => f.write_str("true"),
            Value::Bool(false) => f.write_str("false"),
            Value::Integer(num) => write!(f, "{}", num),
            Value::String(s) => s.fmt(f),
            Value::Path(p) => p.display().fmt(f),
            Value::Attrs(attrs) => attrs.total_fmt(f, set),
            Value::List(list) => list.total_fmt(f, set),
            // TODO: fancy REPL display with position
            Value::Closure(_) => f.write_str("<LAMBDA>"),
            Value::Builtin(builtin) => builtin.fmt(f),

            // Nix prints floats with a maximum precision of 5 digits
            // only. Except when it decides to use scientific notation
            // (with a + after the `e`, and zero-padded to 0 digits)
            Value::Float(num) => total_fmt_float(*num, f),

            // internal types
            Value::AttrNotFound => f.write_str("internal[not found]"),
            Value::Blueprint(_) => f.write_str("internal[blueprint]"),
            Value::DeferredUpvalue(_) => f.write_str("internal[deferred_upvalue]"),
            Value::UnresolvedPath(_) => f.write_str("internal[unresolved_path]"),
            Value::Json(..) => f.write_str("internal[json]"),
            Value::FinaliseRequest(_) => f.write_str("internal[finaliser_sentinel]"),

            // Delegate thunk display to the type, as it must handle
            // the case of already evaluated or cyclic thunks.
            Value::Thunk(t) => t.total_fmt(f, set),
            Value::Catchable(_) => panic!("total_fmt() called on a CatchableErrorKind"),
        }
    }
}

impl From<bool> for Value {
    fn from(b: bool) -> Self {
        Value::Bool(b)
    }
}

impl From<i64> for Value {
    fn from(i: i64) -> Self {
        Self::Integer(i)
    }
}

impl From<f64> for Value {
    fn from(i: f64) -> Self {
        Self::Float(i)
    }
}

impl From<PathBuf> for Value {
    fn from(path: PathBuf) -> Self {
        Self::Path(Box::new(path))
    }
}

fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
    ErrorKind::TypeError {
        expected,
        actual: actual.type_of(),
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::mem::size_of;

    #[test]
    fn size() {
        assert_eq!(size_of::<Value>(), 16);
    }

    mod floats {
        use crate::value::total_fmt_float;

        #[test]
        fn format_float() {
            let ff = [
                (0f64, "0"),
                (1.0f64, "1"),
                (-0.01, "-0.01"),
                (5e+22, "5e+22"),
                (1e6, "1e+06"),
                (-2E-2, "-0.02"),
                (6.626e-34, "6.626e-34"),
                (9_224_617.445_991_227, "9.22462e+06"),
            ];
            for (n, expected) in ff.iter() {
                let mut buf = String::new();
                let res = total_fmt_float(*n, &mut buf);
                assert!(res.is_ok());
                assert_eq!(
                    expected, &buf,
                    "{} should be formatted as {}, but got {}",
                    n, expected, &buf
                );
            }
        }
    }
}