amb ( seq -- elt )
Simple backtracking non-determinism

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Vocabulary
backtrack

Inputs

seq	the alternatives

Outputs

elt	one of the alternatives

Word description
The amb (ambiguous) word saves the state of the current computation (through the continuations vocabulary) and returns the first alternative. When fail is invoked, the saved state will be restored and the next alternative will be returned. When there are no more alternatives, fail will go up one level to the location of the previous amb call. If there are no more calls up the chain, an error will be signalled.

See also
fail, cut-amb

Definition

USING: backtrack.private sequences ;

IN: backtrack

: amb ( seq -- elt ) [ fail f ] [ unsafe-amb ] if-empty ; inline