Package org.organicdesign.fp.xform

Class Xform<A>

  • All Implemented Interfaces:
    java.lang.Iterable<A>, UnmodIterable<A>, Transformable<A>
    public abstract class Xform<A>
extends java.lang.Object
implements UnmodIterable<A>
    An immutable description of operations to be performed (a transformation, transform, or x-form). When fold() (or another terminating function) is called, the Xform definition is "compiled" into a one-time mutable transformation which is then carried out. This allows certain performance shortcuts (such as doing a drop with index addition instead of iteration) and also hides the mutability otherwise inherent in a transformation. Xform is an abstract class. Most of the methods on Xform produce immutable descriptions of actions to take at a later time. These are represented by ___Desc classes. When fold() is called (or any of the helper methods that wrap it), that produces a result by first stringing together a bunch of Operations (____Op classes) and then "running" them. This is analogous to compiling a program and running it. The ____Desc classes are like the immutable source, the ____Op classes like the op-codes it's compiled into. Special thanks to Nathan Williams for pointing me toward separating the mutation from the description of a transformation. Also to Paul Phillips (@extempore2) whose lectures provided an outline for what was ideal and also what was important. All errors are my own. -Glen 2015-08-30

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description
      protected static class  Xform.RunList
      A RunList is a list of Operations "compiled" from an Xform.

    • Field Summary

      Fields 
      Modifier and Type Field Description
      static Xform EMPTY  

    • Method Summary

      All Methods Static Methods Instance Methods Abstract Methods Concrete Methods 
      Modifier and Type Method Description
      Xform<A> concat​(java.lang.Iterable<? extends A> list)
      Add items to the end of this Transformable (precat() adds to the beginning)
      Xform<A> drop​(long n)
      The number of items to drop from the beginning of the output.
      Xform<A> dropWhile​(Fn1<? super A,​java.lang.Boolean> predicate)
      The number of items to drop from the beginning of the output.
      static <T> Xform<T> empty()  
      Xform<A> filter​(Fn1<? super A,​java.lang.Boolean> f)
      Return only the items for which the given predicate returns true.
      <B> Xform<B> flatMap​(Fn1<? super A,​java.lang.Iterable<B>> f)
      Transform each item into zero or more new items using the given function.
      <B> B fold​(B ident, Fn2<? super B,​? super A,​B> reducer)
      Provides a way to collect the results of the transformation.
      <G,​B>
      Or<G,​B>      		 foldUntil​(G accum, Fn2<? super G,​? super A,​B> terminator, Fn2<? super G,​? super A,​G> reducer)
      Thit implementation should be correct, but could be slow in the case where previous operations are slow and the terminateWhen operation is fast and terminates early.
      UnmodIterator<A> iterator()
      A one-time use, mutable, not-thread-safe way to get each value of the underling collection in turn.
      <B> Xform<B> map​(Fn1<? super A,​? extends B> f)
      Transform each item into exactly one new item using the given function.
      static <T> Xform<T> of​(java.lang.Iterable<? extends T> list)
      Static factory methods
      Xform<A> precat​(java.lang.Iterable<? extends A> list)
      Add items to the beginning of this Transformable ("precat" is a PREpending version of conCAT).
      Xform<A> take​(long numItems)
      Return only the first n items.
      Xform<A> takeWhile​(Fn1<? super A,​java.lang.Boolean> f)
      Return items from the beginning until the given predicate returns false.
      protected abstract Xform.RunList toRunList()  

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

      • Methods inherited from interface java.lang.Iterable

        forEach, spliterator

    • Field Detail

      • EMPTY

        public static final Xform EMPTY

    • Method Detail

      • empty

        public static <T> Xform<T> empty()

      • of

        public static <T> Xform<T> of​(java.lang.Iterable<? extends T> list)
        Static factory methods

      • iterator

        public UnmodIterator<A> iterator()
        Description copied from interface: UnmodIterable
        A one-time use, mutable, not-thread-safe way to get each value of the underling collection in turn. I experimented with various thread-safe alternatives, but the JVM is optimized around iterators so this is the lowest common denominator of collection iteration, even though iterators are inherently mutable.
        Specified by:
        iterator in interface java.lang.Iterable<A>
        Specified by:
        iterator in interface UnmodIterable<A>

      • concat

        public Xform<A> concat​(java.lang.Iterable<? extends A> list)
        Description copied from interface: UnmodIterable
        Add items to the end of this Transformable (precat() adds to the beginning)
        Specified by:
        concat in interface Transformable<A>
        Specified by:
        concat in interface UnmodIterable<A>
        Parameters:
        list - the items to add
        Returns:
        a new Transformable with the items added.

      • precat

        public Xform<A> precat​(java.lang.Iterable<? extends A> list)
        Description copied from interface: UnmodIterable
        Add items to the beginning of this Transformable ("precat" is a PREpending version of conCAT).
        Specified by:
        precat in interface Transformable<A>
        Specified by:
        precat in interface UnmodIterable<A>
        Parameters:
        list - the items to add
        Returns:
        a new Transformable with the items added.

      • drop

        public Xform<A> drop​(long n)
        The number of items to drop from the beginning of the output.
        Specified by:
        drop in interface Transformable<A>
        Specified by:
        drop in interface UnmodIterable<A>
        Parameters:
        n - the number of items at the beginning of this Transformable to ignore
        Returns:
        a Transformable with the specified number of items ignored.

      • dropWhile

        public Xform<A> dropWhile​(Fn1<? super A,​java.lang.Boolean> predicate)
        The number of items to drop from the beginning of the output.
        Specified by:
        dropWhile in interface Transformable<A>
        Specified by:
        dropWhile in interface UnmodIterable<A>
        Parameters:
        predicate - the predicate (test function)
        Returns:
        a Transformable with the matching leading items ignored.

      • fold

        public <B> B fold​(B ident,
                  Fn2<? super B,​? super A,​B> reducer)
        Provides a way to collect the results of the transformation.
        Specified by:
        fold in interface Transformable<A>
        Specified by:
        fold in interface UnmodIterable<A>
        Parameters:
        ident - the accumulator and starting value. This will be passed to the function on the first iteration to be combined with the first member of the underlying data source. For some operations you'll need to pass an identity, e.g. for a sum, pass 0, for a product, pass 1 as this parameter.
        reducer - combines each value in the list with the result so far. The initial result is u.
        Returns:
        an eagerly evaluated result which could be a single value like a sum, or a collection.

      • foldUntil

        public <G,​B> Or<G,​B> foldUntil​(G accum,
                                           Fn2<? super G,​? super A,​B> terminator,
                                           Fn2<? super G,​? super A,​G> reducer)
        Thit implementation should be correct, but could be slow in the case where previous operations are slow and the terminateWhen operation is fast and terminates early. It actually renders items to a mutable List, then runs through the list performing the requested reduction, checking for early termination on the result. If you can to a takeWhile() or take() earlier in the transform chain instead of doing it here, always do that. If you really need early termination based on the *result* of a fold, and the operations are expensive or the input is huge, try using a View instead. If you don't care about those things, then this method is perfect for you. Normally you want to terminate by doing a take(), drop(), or takeWhile() before you get to the fold, but if you need to terminate based on the complete result so far, you can provide your own termination condition to this version of fold(). This function can do anything a loop can do. One use case is to accumulate a map and stop if it finds a duplicate key, before overwriting that element in the map. It could then return the map so far, an error, or whatever you like.
        Specified by:
        foldUntil in interface Transformable<A>
        Specified by:
        foldUntil in interface UnmodIterable<A>
        Parameters:
        accum - the accumulator and starting value. This will be passed to the function on the first iteration to be combined with the first member of the underlying data source. For some operations you'll need to pass an identity, e.g. for a sum, pass 0, for a product, pass 1 as this parameter.
        terminator - return null to continue processing. Return non-null to terminate the foldUntil and return Or.bad of this value. This function is called at the beginning of each "loop", thus it's first called with the original value of accum and the first item to process. Returning non-null immediately will prevent the reducer from ever being called.
        reducer - combines each value in the list with the result so far. The initial result is u.
        Returns:
        an Or where the Or.good() is an eagerly evaluated result and Or.bad() is whatever terminateWhen returned.

      • filter

        public Xform<A> filter​(Fn1<? super A,​java.lang.Boolean> f)
        Description copied from interface: UnmodIterable
        Return only the items for which the given predicate returns true.
        Specified by:
        filter in interface Transformable<A>
        Specified by:
        filter in interface UnmodIterable<A>
        Parameters:
        f - a function that returns true for items to keep, false for items to drop
        Returns:
        a Transformable of only the filtered items.

      • flatMap

        public <B> Xform<B> flatMap​(Fn1<? super A,​java.lang.Iterable<B>> f)
        Description copied from interface: UnmodIterable
        Transform each item into zero or more new items using the given function. One of the two higher-order functions that can produce more output items than input items. fold is the other, but flatMap is lazy while fold is eager.
        Specified by:
        flatMap in interface Transformable<A>
        Specified by:
        flatMap in interface UnmodIterable<A>
        Parameters:
        f - yields a Transformable of 0 or more results for each input item.
        Returns:
        a lazily evaluated collection which is expected to be larger than the input collection. For a collection that's the same size, map() is more efficient. If the expected return is smaller, use filter followed by map if possible, or vice versa if not.

      • map

        public <B> Xform<B> map​(Fn1<? super A,​? extends B> f)
        Description copied from interface: UnmodIterable
        Transform each item into exactly one new item using the given function.
        Specified by:
        map in interface Transformable<A>
        Specified by:
        map in interface UnmodIterable<A>
        Parameters:
        f - a function that returns a new value for any value in the input
        Returns:
        a Transformable of the same size as the input (may contain duplicates) containing the return values of the given function in the same order as the input values.

      • take

        public Xform<A> take​(long numItems)
        Description copied from interface: UnmodIterable
        Return only the first n items.
        Specified by:
        take in interface Transformable<A>
        Specified by:
        take in interface UnmodIterable<A>
        Parameters:
        numItems - the maximum number of items in the returned view.
        Returns:
        a Transformable containing no more than the specified number of items.

      • takeWhile

        public Xform<A> takeWhile​(Fn1<? super A,​java.lang.Boolean> f)
        Description copied from interface: UnmodIterable
        Return items from the beginning until the given predicate returns false.
        Specified by:
        takeWhile in interface Transformable<A>
        Specified by:
        takeWhile in interface UnmodIterable<A>
        Parameters:
        f - the predicate (test function)
        Returns:
        a lazy transformable containing the longest un-interrupted run of items, from the beginning of the transformable, that satisfy the given predicate. This could be 0 items to the entire transformable.