public void setKey(FieldValue key) {
        if ( ! type.getKeyType().isAssignableFrom(key.getDataType()))
            throw new IllegalArgumentException("Got " + key.getDataType() + " but require " + type.getKeyType());
        this.key = key;
    }
A field value which represents a map entry. Used to access map entries in for_each.

@author bratseth
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/MapEntryFieldValue.java