67 lines
2.6 KiB
Markdown
67 lines
2.6 KiB
Markdown
# Let's Lens
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Let's Lens presents a series of exercises, in a similar format to
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[the NICTA functional programming course material](http://github.com/NICTA/course).
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The subject of the exercises is around the concept of lenses, initially proposed
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by Foster et al., to solve the view-update problem of relational databases.
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The theories around lenses have been advanced significantly in recent years,
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resulting in a library, implemented in Haskell, called `lens`.
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http://hackage.haskell.org/package/lens
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The exercises take into account various possible goals. For example, if you wish
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to study the history of lenses, then build up to the most recent theories, it is
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best to start at the `Lets.GetSetLens` module. If you wish to derive the
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structure of lenses from first principles, then derive the more modern theories,
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start at the `Lets.Lens` module.
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Exercises can be recognised by filling in a function body that has a placeholder
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of `error "todo: <function-name>"`.
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----
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### Exercise modules
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##### `Lets.GetSetLens`
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This module presents a series of exercises, representing lenses as a traditional
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pair of "`get` and `set`" functions. This representation may be beneficial as it
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easily appeals to an intuition of "what a lens is", however, it is outdated.
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These exercises are useful to gain an initial understanding of the problems that
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lenses solve, as well as to gain an insight into the history of lenses and how
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the theories have developed over time.
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##### `Lets.StoreLens`
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This series of exercises is similar to `Lets.GetSetLens`, however, using a
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slightly altered representation of a lens, based on the `Store` comonad, which
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fuses the typical `get` and `set` operations into a data structure. This
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representation is described in detail in
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*Morris, Tony. "Asymmetric Lenses in Scala." (2012).*
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##### `Lets.OpticPolyLens`
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This series of exercises introduces a new representation of lenses, first
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described by Twan van Laarhoven. This representation also introduces a
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generalisation of lenses to permit *polymorphic update* of structures.
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##### `Lets.Lens`
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This series of exercises starts at first principles to derive the concept of a
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lens, as it was first described by Twan van Laarhoven. The derivation then goes
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on to described other structures to solve various practical problems such as
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*multi-update* and *partial update*.
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This representation presents a generalisation, permitting *polymorphic update*
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over structures. After lenses are derived, further concepts are introduced, such
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as `Fold`s, `Traversal`s and `Prism`s.
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----
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### Credits
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* Edward Kmett on the [derivation of lenses](https://github.com/ekmett/lens/wiki/Derivation)
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