?either ( x left pred -- Either-x )

Vocabulary

monadics

Inputs

x	an object
left	a fallback value
pred	a quotation of type ( x -- bool )

Outputs

Either-x

an Either

Word description

Calls pred on x and either raises the original value to a Right or replaces it with the fallback value as a Left.

Examples

USING: math monadics prettyprint ; 90125 "Not a number." [ number? ] ?either .
T{ Right { value 90125 } }

USING: math monadics prettyprint ; "Hello!" "Not a number." [ number? ] ?either .
T{ Left { value "Not a number." } }

Definition

USING: kernel ;

IN: monadics

: ?either ( x left pred -- Either-x )
    overd call [ drop right ] [ nip left ] if ; inline