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@node Lenses
@section Lenses
Provided by the module @code{(hnh util lens)}
A lens system for Scheme.
@subsection Overview
@subsection Using Lenses
@defun modify container lens f
Returns a new object, with the value focused by @var{lens} replaced by
the result of calling @var{f}, with the old value, and the extra
arguments given.
@end defun
@defun set container lens value
Like modify, but sets a fixed value.
@end defun
@defun get container lens
Get the value focused by lens. Only a single value can me focused.
@end defun
@defun traversed container lens
Similar to @code{get}, but returns a list of all focused elements.
Note that this ``runs'' the lens for each elemement, (where ``modify''
doesn't). So use with caution.
@end defun
@defun lens-compose lenses ...
@defunx compose-lens lenses ...
Creates a new lens, which is the compound from all the given lenses.
Lenses composes left to right, so earlier lenses in @var{lenses} are
applied earlier.
@end defun
@subsection Built in Lenses
@deffn {Lens} car*
@deffnx {Lens} cdr*
Focuses the first or second element of a pair.
@end deffn
@deffn {Lens} ref idx
Focuses the element at index @var{idx} in a list.
@end deffn
@deffn {Lens} identity-lens
A lens focusing the given object. Showing the value returns the root
value, and setting the value just returns the new value.
@end deffn
@deffn {Lens} focus-matching predicate
A lens focusing each element of a list where @var{predicate} returns
true. This may be multiple elements.
@end deffn
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