?unless ( ..a default cond: ( ..a default -- ..a new/f ) false: ( ..a default -- ..b ) -- ..b )

?unless ( ..a default cond: ( ..a default -- ..a new/f ) false: ( ..a default -- ..b ) -- ..b )
Factor handbook » The language » Combinators » Conditional combinators

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Vocabulary
kernel

Inputs

default	an object
cond	an object
false	an object

Outputs
None

Word description
Calls cond on the default object and if cond outputs a new object. Otherwise, calls false with the old object.

Examples
Look up an existing word or make an error pair:

USING: arrays definitions kernel math prettyprint sequences vocabs.parser ; "+" [ search ] [ "not found" 2array ] ?unless .
+

Try to look up a word that doesn't exist:

USING: arrays definitions kernel math prettyprint sequences vocabs.parser ; "+++++" [ search ] [ "not found" 2array ] ?unless .
{ "+++++" "not found" }