Category theory/Natural transformation

< Category theory

Categories: Theoretical foundations

 map even $ maybeToList $ Just 5

yields the same as

 maybeToList $ fmap even $ Just 5

yields: both yield

 [False]

In the followings, this example will be used to illustrate the notion of natural transformation. If the examples are exaggerated and/or the definitions are incomprehensible, try #External links.

Contents 1 Definition 2 Example: maybeToList 2.1 Vertical arrows: sides of objects 2.1.1 Left: side of X object 2.1.2 Right: side of Y object 2.2 Horizontal arrows: sides of functors 2.2.1 Side of Φ functor 2.2.2 Side of Ψ functor 2.3 Commutativity of the diagram 2.4 Remarks 3 Operations 3.1 Mixed 3.1.1 Functor and natural transformation 3.1.2 Natural transformation and functor 4 External links

1 Definition

• Let , denote categories.
• Let be functors.
• Let . Let .

Let us define the natural transformation. It associates to each object of a morphism of in the following way (usually, not sets are discussed here, but proper classes, so I do not use term “function” for this mapping):

• . We call η_A the component of η at A.
• 

Thus, the following diagram commutes (in ):

2 Example: maybeToList

As already mentioned

 map even $ maybeToList $ Just 5

yields the same as

 maybeToList $ fmap even $ Just 5

yields: both yield

 [False]

This example will be shown in the light of the above definition in the followings.

2.1 Vertical arrows: sides of objects

… showing how the natural transformation works.

maybeToList :: Maybe a -> [a]

2.1.1 Left: side of X object

	maybeToList :: Maybe Int -> [Int]
Nothing	[]
Just 0	[0]
Just 1	[1]

2.1.2 Right: side of Y object

	maybeToList :: Maybe Bool -> [Bool]
Nothing	[]
Just True	[True]
Just False	[False]

2.2 Horizontal arrows: sides of functors

 even :: Int -> Bool

2.2.1 Side of Φ functor

	fmap even:: Maybe Int -> Maybe Bool
Nothing	Nothing
Just 0	Just True
Just 1	Just False

2.2.2 Side of Ψ functor

	map even:: [Int] -> [Bool]
[]	[]
[0]	[True]
[1]	[False]

2.3 Commutativity of the diagram

both paths span between

	Maybe Int -> [Bool]
	map even . maybeToList	maybeToList . fmap even
Nothing	[]	[]
Just 0	[True]	[True]
Just 1	[False]	[False]

2.4 Remarks

• even has a more general type (Integral a => a -> Bool) than described here
• Words “side”, “horizontal”, “vertical”, “left”, “right” serve here only to point to the discussed parts of a diagram, thus, they are not part of the scientific terminology.
• If You want to modify the commutative diagram, see its source code (in LaTeX using amscd).

3 Operations

3.1 Mixed

The “mixed” operations described below will be important also in understanding the definition of “monad” concept in category theory.

3.1.1 Functor and natural transformation

Let us imagine a parser library, which contains functions for parsing a form. There are two kinds of cells:

• containing data which are optional (e.g. name of spouse)
• containing data which consist of an enumeration of items (e.g. names of acquired languages)

 spouse :: Parser (Maybe String)
 languages :: Parser [String]

Let us imagine we have any processing (storing, archiving etc.) function which processes lists (or any other reason which forces us to convert our results to list format and exclude any Maybe's). (Perhaps, all this example is impractical and exaggerated, because in real life we should solve the whole thing in other ways.)

Thus, we want to build a parser combinator (we could notate it graphically with something like ) which converts a “zero-ore-one-occurrence” like parser to a “zero-or-one-or-many-occurrences” like parser.

We can convert Maybe to list with maybeToList But if we want to do something similar with a parser on Maybe's to achieve a parser on list, then maybeToList is not enough alone, we must fmap it. E.g. if we want to convert a parser like spouse to be of the same type as languages:

 fmap maybeToList spouse

Let us see the types: We start with

 spouse :: Parser (Maybe String)

Λ(Φ(X))

or using notion of composing functors

(ΛΦ)(X)

We want to achieve

 fmap maybeToList spouse :: Parser [String]

Λ(Ψ(X))

(ΛΨ)(X)

thus we can infer

 fmap maybeToList :: Parser (Maybe [String]) -> Parser [String]

In fact, we have a new “datatype converter”: converting not Maybe's to lists, but parser on Maybe to Parser on list. Let us notate the corresponding natural transformation with Λη:

To each we associate

(Λη)_X = Λ(η_X)

Summary:

Let be categories

functors

functor

natural transformation

Then let us define a new natural transformation:

(Λη)_X = Λ(η_X)

3.1.2 Natural transformation and functor

Let be categories

functor

functors

natural transformation

Then let us define a new natural transformation:

(ηΔ)_X = η_Δ(X)

It can be illustrated by Haskell examples, too. Understanding it is made harder (easier?) by the fact that Haskell's type inference “(dis)solves” the main point, thus there is no “materialized” manifestation of it.

convert :: Maybe (Term a) -> [Term a]

Unlike seen at , the definition of this converter will not show much novelty:

convert = maybeToList

the most interesting thing is done automatically by type inference.

4 External links

• The corresponding HaWiki article is not migrated here yet, so You can see it for more information.
• Wikipedia's Natural transformation article
• Toposes, Triples and Theories written by Michael Barr and Charles Wells.

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