The abstract type of modifiers, parametrized by the type of hook labels. See hook for hook labels. One can largely ignore hook labels unless hook is used.

The idea is to construct a modifier using builders in Language and then execute it using Modifier.S.modify.

Checking equality.

all keeps the content of the current tree. The Modifier.S.Perform.not_found effect will be performed if the tree is empty (nothing to keep). To avoid the emptiness checking, use the identity modifier id. This is equivalent to only [].

id is seq [], which keeps the content of the current tree. This is different from all because, unlike all, id does not check whether the tree is empty.

• since 5.0.0

only path keeps the subtree rooted at path. Bindings outside the subtree are dropped. The Modifier.S.Perform.not_found effect will be performed if the subtree is empty (nothing to keep).

in_ path m runs the modifier m on the subtree rooted at path. Bindings outside the subtree are kept intact.

For example, suppose we have two bindings x.a and y. in_ ["x"] all will keep both x.a and y, while only ["x"] will keep x.y but drop y.

none drops everything. The Modifier.S.Perform.not_found effect will be performed if the tree was already empty (nothing to drop). To avoid the emptiness checking, use the empty modifier union [].

except p drops the subtree rooted at p. The Modifier.S.Perform.not_found effect will be performed if the subtree was already empty (nothing to drop). This is equivalent to in_ p none.

renaming path path' relocates the subtree rooted at path to path'. The existing bindings with the prefix path' (if any) will be dropped. The Modifier.S.Perform.not_found effect will be performed if the subtree was empty (nothing to move).

seq [m0; m1; m2; ...; mn] runs the modifiers m0, m1, m2, ..., mn in order. In particular, seq [] is the identity modifier id.

union [m0; m1; m2; ...; mn] calculates the union of the results of individual modifiers m0, m1, m2, ..., mn.

In particular, union [] is the empty modifier; unlike none, union [] will not check whether the tree was already empty.

The Modifier.S.Perform.shadow effect will be performed to resolve name conflicts while calculating the union, assuming that the results from a later modifier are intended to shadow the results from an earlier modifier.

hook h applies the hook labelled h to the entire tree by performing the Modifier.S.Perform.hook effect.

dump dump_hook m dumps the internal representation of m for debugging, where dump_hook is the ugly printer for hook labels (see hook).