(that: ConfigProvider): (self: ConfigProvider) => ConfigProvider
(self: ConfigProvider, that: ConfigProvider): ConfigProviderReturns a provider that falls back to that when self returns undefined
for a path.
When to use
Use to layer multiple config sources, such as env vars plus a defaults file, or provide partial overrides on top of a base config.
Details
Each provider keeps its own path transformations. If the combined provider is later transformed with mapInput or nested, the transformation is applied to both sides.
Gotchas
The fallback only runs when the path is not found (undefined). A
SourceError from self is not caught; it propagates immediately.
Example (Falling back to a default provider)
import { ConfigProvider } from "effect"
const envProvider = ConfigProvider.fromEnv({
env: { HOST: "prod.example.com" }
})
const defaults = ConfigProvider.fromUnknown({ HOST: "localhost", PORT: "3000" })
const combined = ConfigProvider.orElse(envProvider, defaults)export const const orElse: {
(that: ConfigProvider): (
self: ConfigProvider
) => ConfigProvider
(
self: ConfigProvider,
that: ConfigProvider
): ConfigProvider
}
Returns a provider that falls back to that when self returns undefined
for a path.
When to use
Use to layer multiple config sources, such as env vars plus a defaults file,
or provide partial overrides on top of a base config.
Details
Each provider keeps its own path transformations. If the combined provider
is later transformed with
mapInput
or
nested
, the
transformation is applied to both sides.
Gotchas
The fallback only runs when the path is not found (undefined). A
SourceError from self is not caught; it propagates immediately.
Example (Falling back to a default provider)
import { ConfigProvider } from "effect"
const envProvider = ConfigProvider.fromEnv({
env: { HOST: "prod.example.com" }
})
const defaults = ConfigProvider.fromUnknown({ HOST: "localhost", PORT: "3000" })
const combined = ConfigProvider.orElse(envProvider, defaults)
orElse: {
(that: ConfigProvider(parameter) that: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
self: ConfigProvider(parameter) self: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
self: ConfigProvider(parameter) self: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
that: ConfigProvider(parameter) that: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
dual<(...args: Array<any>) => any, (self: ConfigProvider, that: ConfigProvider) => ConfigProvider>(arity: 2, body: (self: ConfigProvider, that: ConfigProvider) => ConfigProvider): ((...args: Array<any>) => any) & ((self: ConfigProvider, that: ConfigProvider) => ConfigProvider) (+1 overload)Creates a function that can be called in data-first style or data-last
(pipe-friendly) style.
When to use
Use to expose one implementation through both direct and pipe-friendly
call styles.
Details
Pass either the arity of the uncurried function or a predicate that decides
whether the current call is data-first. Arity is the common case. Use a
predicate when optional arguments make arity ambiguous.
Example (Selecting data-first or data-last style by arity)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(2, (self, that) => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Defining overloads with call signatures)
import { Function, pipe } from "effect"
const sum: {
(that: number): (self: number) => number
(self: number, that: number): number
} = Function.dual(2, (self: number, that: number): number => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Selecting data-first or data-last style with a predicate)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(
(args) => args.length === 2,
(self, that) => self + that
)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
dual(
2,
(self: ConfigProvider(parameter) self: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
that: ConfigProvider(parameter) that: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
function makeOrElse(
first: ConfigProvider,
second: ConfigProvider
): ConfigProvider
self: ConfigProvider(parameter) self: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
that: ConfigProvider(parameter) that: {
load: (path: Path) => Effect.Effect<Node | undefined, SourceError>;
state: ProviderState;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}