69 lines
1.7 KiB
Markdown
69 lines
1.7 KiB
Markdown
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title = "`struct`"
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weight = 1
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+++
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Defines a record type (a "pure product"). This is a specialized form of
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a `data` definition. The same field name may occur in more than one
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type.
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```
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topDefn ::= struct typeId {tyVarId }= { { fieldDef ; }} [ derive ]
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fieldDef ::= fieldId :: type
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```
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Example:
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```
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struct Proc = { pc :: Addr; rf :: RegFile; mem :: Memory }
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struct Coord = { x :: Int; y :: Int }
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```
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Section [5.6](fixme) describes how to construct values of a
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`struct` type. A field of a `struct` type can be extracted either
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directly using "dot" notation (section
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[5.7](fixme)) or using pattern matching (section
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[6.3](fixme)).
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### Tuples {#sec-tuple-type}
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One way to group multiple values together is to use a `data` definition
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in which a constructor has multiple fields.
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However, there is a built-in notation for a common form of grouping,
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called "tuples". To group two (or more) values together the Prelude
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contains a type, `PrimPair`, for which there is syntactic sugar for type
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expressions, value expressions, and patterns.
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The type has the following definition
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```hs
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struct PrimPair a b = { fst :: a; snd :: b } deriving (Eq, Bits, Bounded)
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```
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For type expressions the following shorthand can be used:
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(a, b) $\equiv$ PrimPair a b
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Or, more generally,
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($t_1$, $t_2$, $\cdots$, $t_n$) $\equiv$ PrimPair $t_1$ (PrimPair $t_2$ ($\cdots$ $t_n$))
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There is a corresponding shorthand for value expressions and patterns:
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(a, b) $\equiv$ PrimPair { fst = a; snd = b }
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There is also special syntax for the empty tuple. It is written "`()`"
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for types, expressions, and patterns. The real type has the following
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definition
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```hs
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struct PrimUnit = { } deriving (Eq, Bits, Bounded)
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```
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