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catchIf

Recovers a typed failure only when a predicate accepts it.

Package version
2.0.0-beta.7
Category
Errors and recovery
Since
1.0.0

Follow one execution

The predicate rejects denied, so the original typed failure is forwarded and cached never starts.

Fx timelinecatchIf(isOffline, cached)catchIf
OperatorcatchIf(isOffline, cached)
Read this diagram

Follow each lane from left to right. Events stacked vertically share a tick; the green cursor marks the current time across every lane.

  • A value
    The text inside the pill is the emitted value.
  • Work starts
    The raised chevron starts an inner run (^ in the source).
  • The run returns
    The vertical bar ends this lane’s run.
  • A cause is delivered
    The exclamation mark belongs to this lane.
  • Work is interrupted
    The cross marks cancellation of this run.
  • Current time
    The line and diamond move together across all lanes.
  • Happening now
    A highlighted event is at the current tick.
  • Still ahead
    Muted, dashed values have not happened yet.
  • Time continues
    The lane’s arrow is not a return marker. An empty stretch can be quiet work that is still running.

Illustrated ticks start at 0. At 1×, one illustrated tick takes one second; captions specify real durations when timing matters. A cause or interruption belongs to its lane, and other work may continue. Scroll horizontally to inspect the rest of a long timeline.

The predicate is evaluated on the first Fail found anywhere in the source Cause. A match starts one lazy fallback and replaces that entire Cause, including any defects or interrupts composed with the Fail. A rejection, or a Cause containing no Fail, forwards the original Cause unchanged. The fallback contributes its errors and services and shares the subscription.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

import { catchIf } from "@typed/fx/Fx";

Signatures

export declare const catchIf: {
    <E, A2, E2, R2>(predicate: (e: E) => boolean, f: (e: E) => Fx<A2, E2, R2>): <A, R>(self: Fx<A, E, R>) => Fx<A | A2, E | E2, R | R2>;
    <A, E, R, A2, E2, R2>(self: Fx<A, E, R>, predicate: (e: E) => boolean, f: (e: E) => Fx<A2, E2, R2>): Fx<A | A2, E | E2, R | R2>;
};

Why

Some typed errors are distinguished by values rather than tags. Predicate recovery handles that subset while retaining E in the result because a rejected error can still escape.

Ownership and lifetime

The predicate is evaluated on the first Fail found anywhere in the source Cause. A match starts one lazy fallback and replaces that entire Cause, including any defects or interrupts composed with the Fail. A rejection, or a Cause containing no Fail, forwards the original Cause unchanged. The fallback contributes its errors and services and shares the subscription.

Examples

import { catchIf } from "@typed/fx/Fx"
import { fail, succeed } from "@typed/fx/Fx"

const recovered = catchIf(fail(404), (status) => status === 404, () => succeed("missing"))

Other public imports

These import paths expose the same declaration. Each page retains its own public name and signature.

Source