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

/// Main _ad-hoc_ / closure-based constructor of `LendingIterator`s.
///
/// It expects the both necessary and sufficient elements for such an impl:
///
///   - a `State`, which will play a role akin to that of `Self` in a manual
///     impl;
///
///   - a `fn next` "method" on it. There is actually a certain level of
///     flexibility gained from this being a closure rather than a stateless
///     associated function.
///
///     For instance, non-lent state (such as an internal mutable counter) can
///     be implicity captured by such a closure, without having to funnel it
///     through the lendable `State`.
///
/// ## Example
///
/**  - ```rust
    use ::lending_iterator::prelude::*;

    struct Person {
        name: String,
        age: u8,
    }

    fn example (person: &mut Person)
      -> impl '_ + LendingIterator
    /* or:
      -> impl '_ + LendingIteratorDyn<Item = HKT!(&str)>
     */
    {
        lending_iterator::from_fn::<HKT!(&str), _, _>(
            person,
            |p| if p.age > 0 {
                Some(&p.name)
            } else {
                None
            },
        )
    }
    ``` */
///
/// ## Remarks
///
/// It can also be viewed as a convenience layer over:
///
/// <code>::lending_iterator::[repeat_mut]\(state\)<br/>    [.filter_map]::\<Item, _\>\(move |\[\], it| next\(it\)\)</code>
///
/**  - ```rust
    use ::lending_iterator::prelude::*;

    struct Person {
        name: String,
        age: u8,
    }

    fn example (person: &mut Person)
      -> impl '_ + LendingIterator
    /* or:
      -> impl '_ + LendingIteratorDyn<Item = HKT!(&str)>
     */
    {
        lending_iterator::repeat_mut(person)
            .filter_map::<HKT!(&str), _>(
                |[], p| if p.age > 0 {
                    Some(&p.name)
                } else {
                    None
                },
            )
    }
    ``` */
///
/// [repeat_mut]: crate::repeat_mut()
/// [.filter_map]: crate::LendingIterator::filter_map
///
/// ### `FromFn`
///
/// The returned `struct` —[`FromFn`]— can also be used directly, to benefit
/// from "named arguments", at the cost of having to provide a `PhantomData`
/// parameter.
pub
fn from_fn<Item, State, Next> (
    state: State,
    next: Next,
) -> FromFn<Item, State, Next>
where
    Item : HKT,
    Next : FnMut(&'_ mut State) -> Option< A!(Item<'_>) >,
{
    FromFn { state, next, _phantom: <_>::default() }
}

/// The <code>impl [LendingIterator]</code> returned by [`from_fn()`].
///
/// ## Example
///
/**  - ```rust
    use ::lending_iterator::prelude::*;

    struct Person {
        name: String,
        age: u8,
    }

    fn example (person: &mut Person)
      -> impl '_ + LendingIterator
    /* or:
      -> impl '_ + LendingIteratorDyn<Item = HKT!(&str)>
     */
    {
        lending_iterator::FromFn::<HKT!(&str), _, _> {
            state: person,
            next: |p| if p.age > 0 {
                Some(&p.name)
            } else {
                None
            },
            _phantom: <_>::default(),
        }
    }
    ``` */
pub
struct FromFn<Item, State, Next>
where
    Item : HKT,
    Next : FnMut(&'_ mut State) -> Option< A!(Item<'_>) >,
{
    /// The state owned by this [`LendingIterator`].
    ///
    ///  - Think of `Self` within a manual implementation of the trait;
    ///
    ///  - Think of [`repeat_mut()`].
    pub
    state: State,

    /// The "`fn next()`" of a "manual implementation of the trait".
    ///
    /// Trick: since it's only required to be a closure, this `Next` closure
    /// can capture state on its own, provided it does not need to lend from it.
    ///
    /// This can lead to slightly more lightweight `FromFn` / `from_fn` calls:
    ///   - put the lent / borrowed state inside `.state`,
    ///   - let the rest of the state be implicitly `move`-captured by this closure.
    pub
    next: Next,

    /// The signature of `fn next` in a `PhantomData`.
    pub
    _phantom: PhantomData<
        fn(&mut State) -> Option<A!(Item<'_>)>,
    >,
}

#[gat]
impl<Item, State, Next>
    LendingIterator
for
    FromFn<Item, State, Next>
where
    Item : HKT,
    Next : FnMut(&'_ mut State) -> Option< A!(Item<'_>) >,
{
    type Item<'next>
    where
        Self : 'next,
    =
        A!(Item<'next>)
    ;

    fn next (self: &'_ mut FromFn<Item, State, Next>)
      -> Option< A!(Item<'_>) >
    {
        let Self { state, next, .. } = self;
        next(state)
    }
}