An effect corresponding to the MonadFail type class.

Effly's MonadFail instance is based on this effect; by having access to Fail, you're able to invoke handle pattern-match failure automatically inside of effect handlers.

Each Fail interpreter's associated carrier has an MonadFail instance based on how it interprets Fail. This means you can use an Fail interpreter to locally gain access to an MonadFail instance inside of application code.

Run a Fail effect purely, by returning Left failureMessage upon a pattern match failure.

FailC has an Alternative instance based on the Alt effect it interprets.

Transform a Fail effect to a Throw effect by providing a function to transform a pattern match failure into an exception.

You can use this in application code to locally get access to a MonadFail instance (since InterpretFailReifiedC has a MonadFail instance based on the Fail effect this interprets).

For example:

  failToThrow (_ -> throw exc) (do { Just a <- pure Nothing; return a})
= throw exc

This has a higher-rank type, as it makes use of InterpretFailReifiedC. This makes failToThrow very difficult to use partially applied. In particular, it can't be composed using ..

If performance is secondary, consider using the slower failToThrowSimple, which doesn't have a higher-rank type. However, you typically don't want to use failToThrowSimple in application code, since failToThrowSimple emits a ReaderThreads threading constraint (see Threaders).

Transform a Fail effect to a NonDet effect by having a pattern match failure be lose.

You can use this in application code to locally get access to a MonadFail instance (since FailToNonDetC has a MonadFail instance based on the Fail effect this interprets).

For example:

  failToNonDet (do { Just a <- pure Nothing; return a})
= lose

Transform a Fail effect to an Alt effect by having a pattern match failure be empty.

You can use this in application code to locally get access to a MonadFail instance (since FailToAltC has a MonadFail instance based on the Fail effect this interprets).

Transform a Fail effect to a Throw effect by providing a function to transform a pattern match failure into an exception.

This is a less performant version of failToThrow that doesn't have a higher-rank type, making it much easier to use partially applied.

Unlike failToThrow, you typically don't want to use this in application code, since this emits a ReaderThreads threading constraint (see Threaders).

ErrorThreads accepts the following primitive effects:

• Regional s
• Optional s (when s is a functor)
• BaseControl b
• Unravel p
• ListenPrim s (when s is a Monoid)
• WriterPrim s (when s is a Monoid)
• ReaderPrim i
• Mask
• Bracket
• Fix

Like InterpretC specialized to interpret Fail, but with a MonadFail instance based on the interpreted Fail.

Like InterpretSimpleC specialized to interpret Fail, but with a MonadFail instance based on the interpreted Fail.