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70 lines
2.8 KiB
ReStructuredText
70 lines
2.8 KiB
ReStructuredText
.. _sect-intro:
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************
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Introduction
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************
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In conventional programming languages, there is a clear distinction
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between *types* and *values*. For example, in `Haskell
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<http://www.haskell.org>`_, the following are types, representing
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integers, characters, lists of characters, and lists of any value
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respectively:
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- ``Int``, ``Char``, ``[Char]``, ``[a]``
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Correspondingly, the following values are examples of inhabitants of
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those types:
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- ``42``, ``’a’``, ``Hello world!``, ``[2,3,4,5,6]``
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In a language with *dependent types*, however, the distinction is less
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clear. Dependent types allow types to “depend” on values — in other
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words, types are a *first class* language construct and can be
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manipulated like any other value. The standard example is the type of
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lists of a given length [1]_, ``Vect n a``, where ``a`` is the element
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type and ``n`` is the length of the list and can be an arbitrary term.
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When types can contain values, and where those values describe
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properties, for example the length of a list, the type of a function
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can begin to describe its own properties. Take for example the
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concatenatation of two lists. This operation has the property that the
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resulting list's length is the sum of the lengths of the two input
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lists. We can therefore give the following type to the ``app``
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function, which concatenates vectors:
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.. code-block:: idris
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app : Vect n a -> Vect m a -> Vect (n + m) a
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This tutorial introduces Idris, a general purpose functional
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programming language with dependent types. The goal of the Idris
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project is to build a dependently typed language suitable for
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verifiable *systems* programming. To this end, Idris is a compiled
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language which aims to generate efficient executable code. It also has
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a lightweight foreign function interface which allows easy interaction
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with external ``C`` libraries.
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Intended Audience
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=================
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This tutorial is intended as a brief introduction to the language, and
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is aimed at readers already familiar with a functional language such
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as `Haskell <http://www.haskell.org>`_ or `OCaml <http://ocaml.org>`_.
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In particular, a certain amount of familiarity with Haskell syntax is
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assumed, although most concepts will at least be explained
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briefly. The reader is also assumed to have some interest in using
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dependent types for writing and verifying systems software.
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Example Code
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============
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This tutorial includes some example code, which has been tested with
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against Idris. These files are available with the Idris distribution,
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so that you can try them out easily. They can be found under
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``samples``. It is, however, strongly recommended that you type
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them in yourself, rather than simply loading and reading them.
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.. [1]
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Typically, and perhaps confusingly, referred to in the dependently
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typed programming literature as “vectors”
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