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6851d40 | 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 | // RUN: %run "%s" > "%t"
// RUN: %OutputCheck --file-to-check "%t" "%s"
// CHECK-L: Hello
// CHECK-NEXT-L: Error: 'name' was not found
// CHECK-NEXT-L: Hello
// CHECK-NEXT-L: Dafny
// CHECK-NEXT-L: Test dummy file
include "../src/dafny/Wrappers.dfy"
module {:options "--function-syntax:4"} Demo {
import opened Dafny.Wrappers
// ------ Demo for Option ----------------------------
// We use Option when we don't need to pass around a reason for the failure,
// ie when just 'None' is sufficient:
class MyMap<K(==), V> {
var m: map<K, V>
constructor () {
m := map[];
}
function Get(k: K): Option<V>
reads this
{
if k in m then Some(m[k]) else None()
}
method Put(k: K, v: V)
modifies this
{
this.m := this.m[k := v];
}
}
method TestMyMap() {
var m := new MyMap<string, string>();
m.Put("message", "Hello");
Greet(m);
m.Put("name", "Dafny");
Greet(m);
}
method Greet(m: MyMap<string, string>) {
var o: Option<string> := m.Get("message");
if o.Some? {
print o.value, "\n";
} else {
print "oops\n";
}
var r: Result<string, string> := FindName(m);
if r.Success? {
print r.value, "\n";
} else {
print "Error: ", r.error, "\n";
}
}
// Sometimes we want to go from Option to Result:
method FindName(m: MyMap<string, string>) returns (res: Result<string, string>) {
// Will return a default error message in case of None:
res := m.Get("name").ToResult("'name' not found");
// We can also match on the option to write a custom error:
match m.Get("name")
case Some(n) => res := Success(n);
case None => res := Failure("'name' was not found");
}
// Propagating failures using :- statements
method GetGreeting(m: MyMap<string, string>) returns (res: Option<string>) {
var message: string :- m.Get("message");
var nameResult := FindName(m);
var name :- nameResult.ToOption();
res := Some(message + " " + name);
}
// ------ Demo for Result ----------------------------
// We use Result when we want to give a reason for the failure:
class MyFilesystem {
var files: map<string, string>
constructor() {
files := map[];
}
// Result<()> is used to return a Result without any data
method WriteFile(path: string, contents: string) returns (res: Result<(), string>)
modifies this
{
if path in files {
files := files[path := contents];
res := Success(()); // `()` is the only value of type `()`
} else {
// The "Result" datatype allows us to give precise error messages
// instead of just "None"
res := Failure("File not found, use 'Create' before");
}
}
method CreateFile(path: string) returns (res: Result<(), string>)
modifies this
{
if path in files {
res := Failure("File already exists");
} else {
files := files[path := ""];
res := Success(());
}
}
method ReadFile(path: string) returns (res: Result<string, string>) {
if path in files {
res := Success(files[path]);
} else {
res := Failure("File not found");
}
}
}
// Propagating failures using :- statements
method CopyFile(fs: MyFilesystem, fromPath: string, toPath: string) returns (res: Result<(), string>)
modifies fs
{
var contents :- fs.ReadFile(fromPath);
var _ :- fs.CreateFile(toPath);
var _ :- fs.WriteFile(toPath, contents);
res := Success(());
}
method TestMyFilesystem() {
var fs := new MyFilesystem();
// Note: these verbose "outcome.Failure?" patterns will soon
// not be needed any more, see https://github.com/dafny-lang/rfcs/pull/1
var outcome: Result<(), string> := fs.CreateFile("test.txt");
if outcome.Failure? {
print outcome.error, "\n";
return;
}
outcome := fs.WriteFile("test.txt", "Test dummy file");
if outcome.Failure? {
print outcome.error, "\n";
return;
}
var fileResult: Result<string, string> := fs.ReadFile("test.txt");
if outcome.Failure? {
print fileResult.error, "\n";
return;
}
if fileResult.Success? {
print fileResult.value, "\n";
}
}
// ------ Demo for Need ----------------------------
// We use Need when something has to be true but we can't prove it statically
// This is a very contrived example
method whatIsCharacterFive(fs: MyFilesystem, fromPath: string) returns (res: Result<char, string>)
modifies fs
{
// Get a string that we can't reason about statically
var contents :- fs.ReadFile(fromPath);
// Dynamically test whether the string is at least 5 characters long, and return a failure if not.
// If we pass this line, Dafny can now assume that the string is long enough.
:- Need<string>(|contents| >= 5, "File contents not long enough.");
// Now we can get the character
var c := contents[4];
return Success(c);
}
// For a method that returns an Outcome, use Outcome.Need
method whatIsCharacterFive'(fs: MyFilesystem, fromPath: string) returns (res: Outcome<string>)
modifies fs
{
// Get a string that we can't reason about statically
var contents: string := *;
// Dynamically test whether the string is at least 5 characters long, and return a failure if not.
// If we pass this line, Dafny can now assume that the string is long enough.
:- Outcome.Need(|contents| >= 5, "File contents not long enough.");
// Now we can get the character
var c := contents[4];
// and do other stuff
return Pass;
}
method Main() {
TestMyMap();
TestMyFilesystem();
}
}
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