Effect Requests

Another way of creating effects in Fx.go is via an effect-request function.

A function of type func(I) Fx[S, O] is said to take an effect-request I and produce an suspended effect Fx[S, O].

An example, is the function func(HttpReq) Fx[HttpService, HttpRes] we saw in the previous chapter.

Using the "length of string" example of the previous chapters, we can use it to model an effect request:

type LenFn = func(string) fx.Fx[fx.Nil, int]

// Code is type annotated for clarity
var lenFx fx.Fx[fx.And[LenFn, fx.Nil], int] = fx.Suspend[LenFn]("hello")

Note that Suspend takes the type of a request-effect function and the request value for it. And yields a suspended effect of type Fx[And[LenFn, Nil], int]. The computation is said to be suspended because it knows not what particular implementation of LenFn should be used, and because of this, LenFn is part of the requirements, along with Nil the ability requirement on the result of LenFn.

Different implementations of LenFn can be provided to the lenFx effect.

var lies LenFn = func(_ string) fx.Fx[fx.Nil, int] {
    return fx.Pure(42)
}
var truth LenFn = func(s string) fx.Fx[fx.Nil, int] {
    return fx.Pure(len(s))
}

var x int = fx.Eval(fx.ProvideLeft(lenFx, lies))
assert(x == 42)

var y int = fx.Eval(fx.ProvideLeft(lenFx, truth))
assert(y == 5)

Notice that by delaying which implementation of LenFn is used, the lenFx program description includes the effect request "hello" and knows the general form of its response Fx[Nil, int], but knows nothing about which particular interpretation of LenFn will be used.