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a574e6af09
notes/TYPES: Rewritten. The old one was obsolete. notes/COMPILER_DESIGN: Fix this up a bit.
71 lines
1.8 KiB
Plaintext
71 lines
1.8 KiB
Plaintext
Mercury's Type System
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---------------------
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The type system is based on a polymorphic many-sorted logic,
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similar to Goedel. But the syntax is different (in some respects better),
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we allow equivalence types, and we allow overloading of function symbols
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defined within the same module (Goedel only allows it if the function symbols
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are defined in different modules).
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There are two categories of builtin types:
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Primitive types: char, integer, float, string. These are automatically
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predefined, there is a special syntax for constants of these types, and
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there are infinitely many constants for some of these types.
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Predicate types: pred, pred(T), pred(T1, T2), ...
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These are used in higher-order code to pass predicates around.
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There are several categories of derived types:
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Discriminated unions: theses encompass both enumerations and
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records, and are defined as follows.
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:- type fruit
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---> apple
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; orange
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; banana
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; pear.
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:- type strange
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---> foo(int)
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; bar(string).
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:- type employee
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---> employee(
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string, % name
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int, % age
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string % department
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).
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:- type tree
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---> empty
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; leaf(int)
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; branch(tree, tree).
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:- type list(T)
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---> []
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; '.'(T, list(T)).
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:- type pair(T1, T2)
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---> T1 - T2.
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Equivalent types: these are type abbreviations. They are defined
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using `==' as follows.
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:- type money == int.
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:- type assoc_list(KeyType, ValueType)
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== list(pair(KeyType, ValueType)).
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Abstract types: these are types whose implementation is hidden.
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The type declarations
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:- type t1.
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:- type t2(T1, T2).
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declare types `t1/0' and `t/2' to be abstract types.
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Such declarations are only useful in the interface section of a module.
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This means that the type names will be exported, but the constructors
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(constants and functors) for these types will not be exported.
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