Fallback

Fallback operators recover from typed failures. Defects and interruptions remain unchanged.

catch

Effect.catch receives the error and returns a fallback Effect. If the source succeeds, the fallback is not evaluated.

Example (Recovering with Another Effect)

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import { Effect } from "effect"
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const primary = Effect.fail("primary unavailable")
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const program = primary.pipe(
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  Effect.catch((error) => Effect.succeed(`fallback: ${error}`)),
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)
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Effect.runSync(program) // => "fallback: primary unavailable"

Use Effect.catchTag, Effect.catchIf, or Effect.catchFilter when only part of the error channel should trigger the fallback.

orElseSucceed

Effect.orElseSucceed replaces any typed failure with a lazily evaluated success value and removes the typed error channel.

Example (Providing a Default Value)

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import { Effect } from "effect"
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const program = Effect.fail("missing").pipe(Effect.orElseSucceed(() => 0))
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Effect.runSync(program) // => 0

This operator handles every typed error. If only absence or one specific error should use the default, narrow the error first with a selective catch operator.

firstSuccessOf

Effect.firstSuccessOf runs alternatives sequentially and stops at the first success. If every effect fails, it propagates the last error.

Example (Trying Prioritized Alternatives)

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import { Effect } from "effect"
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const program = Effect.firstSuccessOf([
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  Effect.fail("primary unavailable"),
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  Effect.succeed("secondary result"),
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  Effect.die("not evaluated"),
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])
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Effect.runSync(program) // => "secondary result"

Passing an empty iterable creates a defect with the message "Received an empty collection of effects".