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/*
* Copyright 2016 Google Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.turbine.binder.env;
import static java.util.Objects.requireNonNull;
import com.google.turbine.binder.sym.Symbol;
import javax.annotation.Nullable;
/** An {@link Env} that chains two existing envs together. */
public class CompoundEnv<S extends Symbol, V> implements Env<S, V> {
private final Env<S, ? extends V> base;
private final Env<S, ? extends V> env;
private CompoundEnv(@Nullable Env<S, ? extends V> base, Env<S, ? extends V> env) {
this.base = base;
this.env = requireNonNull(env);
}
@Override
public V get(S sym) {
V result = env.get(sym);
if (result != null) {
return result;
}
return base != null ? base.get(sym) : null;
}
/** A chainable compound env with a single entry. */
public static <S extends Symbol, V> CompoundEnv<S, V> of(Env<S, ? extends V> env) {
return new CompoundEnv<>(null, env);
}
/** Adds an env to the chain. */
public CompoundEnv<S, V> append(Env<S, ? extends V> env) {
return new CompoundEnv<>(this, env);
}
}
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