?when ( ..a default cond: ( ..a default -- ..a new/f ) true: ( ..a new -- ..a x ) -- ..a default/x )
Factor handbook » The language » Combinators » Conditional combinators

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Vocabulary
kernel

Inputs

default	an object
cond	a quotation with stack effect ( ..a default -- ..a new/f )
true	a quotation with stack effect ( ..a new -- ..a x )

Outputs

default/x

default or x

Word description
Calls cond on the default object and if cond outputs a new object then the true quotation is called with that new object. Otherwise, leaves the old object on the stack.

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

USING: arrays definitions kernel math prettyprint sequences vocabs.parser ; "+" [ search ] [ where first ] ?when .
"resource:core/math/math.factor"

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

USING: arrays definitions kernel math prettyprint sequences vocabs.parser ; "+++++" [ search ] [ where first ] ?when .
"+++++"

See also
?if, ?unless

Definition

IN: kernel

: ?when
    ( ..a default cond: ( ..a default -- ..a new/f ) true: ( ..a new -- ..a x ) -- ..a default/x )
    [ ?transmute ] dip when ; inline