go-monads
go-monads is a library that implements basic Haskell monads, based on Go 2 Generics Draft.
Table of Contents
Maybe(T)
Maybe represents type that can either have value or be empty
Create Maybe
x := maybe.Return(int)(5)
or
val := new(int)
x := maybe.OfNullable(int)(val)
or
x := maybe.Empty(struct{})()Transform Maybe into new value
x := maybe.Return(int)(5)
fmt.Println(Map(int, string)(x, fmt.Sprint))
or
func divideBy(x int, y int) Maybe(int) {
if y == 0 {
return Empty(int)()
}
return Return(int)(x/y)
}
four := maybe.Map(int, int)(divideBy(6, 3), func (x int) int { return x * 2 }) //four equals Just{obj: 4}
nothing := maybe.Map(int, int)(divideBy(6, 0), func (x int) int { return x * 2 }) //nothing equals Nothing{}
or
maybe.FlatMap(int, int)(divideBy(6, x), func (x int) { return divideBy(x, y) }) //x, y are some unknown integers, might be zerosIO(T)
IO encodes computation that potentially contains side effects as pure value
Create IO
//countNumberOfBytesInFile is an effectfull computation that returns number of bytes in file
func countNumberOfBytesInFile(f os.File) int {
fi, _ := f.Stat()
return fi.Size()
}
x := io.Return(int)(countNumberOfBytesIntFile(someFile))Transform IO into new value
func printVal(type T)(val T) IO(T) {
return Return(int)(func () {
fmt.Println(val)
return val
})
}
func main() {
io.FlatMap(int, int)(countNumberOfBytesInFile(), printVal).UnsafePerformIO()
}Either(L, R)
Either represents value that can be of one of two types
Create Either
x := either.AsRight(int)(5)
y := either.AsLeft(string)("xyz")Example of usage
For example, we want to check whether file contains substring 'go'
Some helper functions
func toString(b []byte) string {
return string(b)
}
func contains(text string) bool {
return strings.Contains(text, "go")
}Standard way
func ContainsGo(reader io.Reader) (bool, error) {
bytes, err:= ioutil.ReadAll(reader)
if err != nil {
return false, err
}
text := toString(bytes)
return contains(text), nil
}ReadAllE - function that call ReaderAll and wrap result to Either
func ReadAllE(reader io.Reader) Either(error, []byte) Generic way
func ContainsGoE(reader io.Reader) Either(error, bool) {
return either.Map(error, string, bool)(either.Map(error, []byte, string)(ReadAllE(reader), toString), contains)
}State(S, A)
Represent function func (S) (A, S), where S is state, A is result
Create State
x := state.Return(int, string)(5)
y := state.Put(int)(5)Transform State into new value
func CountZeroes(numbs []int) int {
counter := state.Put(int)(0)
for _, x := range numbs {
state := state.bindI(int, Unit, int)(counter, state.Get(int)())
counter = state.FlatMap(int, int, int)(state, func(c int) { return state.Put(int)(c + 1) })
}
return counter.RunState(0)
}
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