/
maybe.hpp
171 lines (145 loc) · 6.61 KB
/
maybe.hpp
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
// Copyright 2012 Justus Sagemüller.
// This file is part of the Lambdalike library.
//This library is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with this library. If not, see <http://www.gnu.org/licenses/>.
#ifndef HASKELLLIKE_CLASS_MAYBE_MONAD
#define HASKELLLIKE_CLASS_MAYBE_MONAD
namespace lambdalike {
template<typename DataT>
class maybe;
template<typename DataT>
maybe<DataT> just(DataT obj);
struct nothing_object{nothing_object(){}};
const nothing_object nothing;
// template class objects of which may or may not contain some given
// data object. Inspired by Haskell's Maybe monad.
//Applications are often very similar to those of std::unique_ptr,
// the main difference being that a maybe is copyable but can't hold
// derived instances.
template<typename DataT>
class maybe {
DataT *obj;
public:
class iterator {
DataT *mobj;
explicit iterator(DataT *init):mobj(init){}
public:
iterator():mobj(nullptr){}
iterator(const iterator &cp):mobj(cp.mobj){}
bool operator!=(const iterator &other)const{return mobj!=other.mobj;}
DataT &operator*() const{return *mobj;}
iterator &operator++(){ mobj=nullptr; return *this; }
friend class maybe;
};
class const_iterator {
const DataT *mobj;
explicit const_iterator(const DataT *init):mobj(init){}
public:
const_iterator():mobj(nullptr){}
const_iterator(const const_iterator &cp):mobj(cp.mobj){}
bool operator!=(const const_iterator &other)const{return mobj!=other.mobj;}
const DataT &operator*() const{return *mobj;}
const_iterator &operator++(){ mobj=nullptr; return *this; }
friend class maybe;
};
iterator begin(){return iterator(obj);}
iterator end(){return iterator();}
const_iterator begin()const{return const_iterator(obj);}
const_iterator end()const{return const_iterator();}
const_iterator c_begin()const{return const_iterator(obj);}
const_iterator c_end()const{return const_iterator();}
bool is_nothing()const{return obj==nullptr;}
void make_nothing(){delete obj; obj=nullptr;}
bool is_just()const{return obj!=nullptr;}
template<typename CpDataT>
void with_just_assign(CpDataT &mdftg)const{if(obj) mdftg=*obj;}
DataT &from_just(){return *obj;}
DataT &operator*(){return *obj;}
const DataT &from_just()const{return *obj;}
const DataT &operator*()const{return *obj;}
template<typename CmpDataT>
bool operator==(const maybe<CmpDataT> &cmp)const{
return is_just()==cmp.is_just() && (is_nothing() || *obj==*cmp.obj); }
template<typename CmpDataT>
bool operator!=(const maybe<CmpDataT> &cmp)const{
return is_just()!=cmp.is_just() || (is_just() && *obj!=*cmp.obj); }
bool operator==(const nothing_object &n)const{return obj==nullptr;}
bool operator!=(const nothing_object &n)const{return obj!=nullptr;}
template<typename MpFnT>
auto fmapped(MpFnT f) const -> maybe<decltype(f(*obj))> {
return obj? just(f(*obj)) : nothing; }
template<typename MonadicFn>
auto operator>>=(MonadicFn f) const -> decltype(f(*obj)) {
return obj? f(*obj) : nothing; }
template<typename MonadicFn>
auto monadicbind_destructive(MonadicFn f) -> decltype(f(*obj)) {
return obj? f(std::move(*obj)) : nothing; }
template<typename ReplaceDT>
auto operator>>(const maybe<ReplaceDT> &r) const -> maybe<ReplaceDT> {
return obj? r : nothing; }
template<typename ReplaceDT>
auto operator>>(maybe<ReplaceDT>&& r) const -> maybe<ReplaceDT> {
return obj? std::move(r) : nothing; }
auto operator>>(const nothing_object &n) const -> maybe<DataT> {
return nothing; }
// old constructor set, flawed
#ifdef USE_ALTERNATIVE_CONSTRUCTORS_FOR_MAYBE_DATATYPE
maybe(const nothing_object &n):obj(nullptr){}
template<typename CpDataT>
explicit maybe(const CpDataT &cobj):obj(new DataT(cobj)){}
template<typename CpDataT>
maybe &operator=(const CpDataT &cobj){delete obj; obj=new DataT(cobj); return *this;}
template<typename CpDataT>
maybe(const maybe<CpDataT> &cp):obj(cp.is_just()?new DataT(cp.from_just()):nullptr){}
template<typename CpDataT>
maybe &operator=(const maybe<CpDataT> &cp){
delete obj; obj = cp.is_just()? new DataT(cp.from_just()) : nullptr; return *this;}
maybe(maybe<DataT> &&mv):obj(mv.obj){mv.obj=nullptr;}
maybe &operator=(maybe<DataT> &&mv) {
delete obj; obj=mv.obj; mv.obj=nullptr; return *this; }
#else
maybe(const nothing_object &n):obj(nullptr){}
explicit maybe(DataT cobj):obj(new DataT(std::move(cobj))){}
maybe &operator=(DataT cobj){delete obj; obj=new DataT(std::move(cobj)); return *this;}
template<typename CpDataT>
maybe(const maybe<CpDataT> &cp):obj(cp.is_just()?new DataT(cp.from_just()):nullptr){}
template<typename CpDataT>
maybe &operator=(const maybe<CpDataT> &cp){
delete obj; obj = cp.is_just()? new DataT(cp.from_just()) : nullptr; return *this;}
maybe(maybe<DataT> &&mv)noexcept :obj(mv.obj) {mv.obj=nullptr;}
maybe &operator=(maybe<DataT> &&mv) noexcept {
delete obj; obj=mv.obj; mv.obj=nullptr; return *this; }
#endif
~maybe(){delete obj;}
};
template<typename DataT>
auto just(DataT obj) -> maybe<DataT> { return maybe<DataT>(std::move(obj)); }
template<typename MpFnT, typename DataT> // represents Haskell's <$> infix
auto operator^(MpFnT f, const maybe<DataT> &m) -> decltype(m.fmapped(f)) {
return m.fmapped(f);
}
template<typename DataT>
auto joined(const maybe<maybe<DataT>> &m) -> maybe<DataT> {
return m.is_just()? m.from_just() : nothing;
}
template<typename DataT>
auto maybe_yes(const std::pair<DataT,bool>& mbcst) -> maybe<DataT> {
return mbcst.second ? just(mbcst.first) : nothing;
}
template<typename DataT>
auto maybe_not(const std::pair<DataT,bool>& mbcst) -> maybe<DataT> {
return !mbcst.second ? just(mbcst.first) : nothing;
}
template<typename DataT, typename MonadicFn>auto
operator>>=(maybe<DataT>&& dsrc, MonadicFn f) -> decltype(f(std::declval<DataT>())) {
return dsrc.monadicbind_destructive(f); }
}
#endif