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Generics.EMGM.Common.Representation | Portability | non-portable | Stability | experimental | Maintainer | generics@haskell.org |
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Description |
Summary: Types and related functions for the representation used in EMGM.
EMGM uses a generic sum-of-products view of datatypes encoded into the
Unit, '(:^:)' (sum), and '(:*:)' (product). Many Haskell datatypes can be
represented in this way. Right-nested sums replace the |, and right-nested
products replace the arguments to a constructor. Units replace constructors
with no arguments.
Since constructors encode more than just a list of arguments, this library
uses ConDescr to store that information. This includes name, arity, record
labels, fixity, and operator precedence. Constructor descriptions are useful
for generic operations such as Read and Show and possibly others.
Generic functions need to convert values between the Haskell datatype and its
structure representation. This is done using the embedding-projection pair,
which is simply a pair a functions for translating between two types.
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Synopsis |
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Structure representation
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The generic sum-of-products view of a Haskell datatype.
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Unit encodes a constructor with no arguments. An analogous standard
Haskell type is ().
| Constructors | Unit | The only value of type Unit (ignoring _|_).
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| Instances | |
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The "sum" encodes 2 constructor alternatives. An analogous standard
Haskell type is Either a b.
| Constructors | L a | Left alternative
| R b | Right alternative
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The "product" encodes 2 constructor arguments. An analogous standard
Haskell type is (a, b).
| Constructors | a :*: b | A pair of arguments
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Constructor description
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Since this library does not have access to the syntax of a data
declaration, it relies on ConDescr for meta-information. It is
important that the ConDescr for a constructor accurately describe the
actual syntax in the declaration. An incorrect description may lead to
bad Read or Show operation.
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A constructor description containing useful meta-information about the
syntax used in the data declaration. This is particularly useful in Read
and Show but may also be helpful in other generic functions.
| Constructors | ConDescr | | conName :: String | Name of the constructor. If it is infix,
don't provide parentheses.
| conArity :: Int | Arity or number of arguments.
| conLabels :: [String] | A list of labels used in record syntax.
They must be declared in the same order as
the data declaration. The list should be
empty if the constructor is not a record.
| conFixity :: Fixity | Infix or not, associativity, precedence.
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The constructor type used in Read and Show to determine how to parse or
print the constructor.
| Constructors | ConStd | Standard (function-type, nonfix)
| ConRec [String] | Record-style (nonfix or infix)
| ConIfx String | Infix (no record syntax)
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Fixity and precedence
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These are used to determine whether a constructor is infix or not and, if
it is infix, what its associativity and precedence are.
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Determine an identifier's status as infix or not. If infix, the
associativity and precedence are also determined.
| Constructors | Nonfix | Not infix. Precedence is same as function application (see appPrec).
| Infix Prec | Non-associative infix with precedence.
| Infixl Prec | Left-associative infix with precedence.
| Infixr Prec | Right-associative Infix with precedence.
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Prec |
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Get the precedence of a fixity value.
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minPrec |
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Maximum precedence: 11
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Precedence for function application: 10
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Precedence for record construction: 11
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Embedding-projection pair
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The Embedding-Projection pair contains two functions for converting between
the datatype and its representation. This pair preserves an isomorphism
(ignoring _|_s) between a datatype and its structure.
| Constructors | EP | | from :: d -> r | Embed a datatype into its representation.
| to :: r -> d | Project datatype from its representation.
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