Data Types
Typing: strong vs. weak, static vs. dynamic. Type declarations, type equivalence, type inference. Subtypes and derived types. Scalar and composite types (arrays, records, variant records). Pointers and References
Readings: Scott, ch. 7
A language can have a mixture of both, like C#, Visual Basic, Alore. Java has a mostly static type system with some runtime checks.
Languages like Ada, C++, Java, ML.
- Variables have types.
- Compilers ensures (at compile time) that type rules are obeyed.
Languages like JavaScript, PHP, Lisp, Ruby.
- Variables do not have types, values do.
- Compilers ensures (at run time) that type rules are obeyed.
- Static is faster. Dynamic typing requires run-time checks.
- Dynamic is more flexible.
- Static is easier to refactor code.
A strongly typed language does not allow variables to be used in a way inconsistent with their types (no loopholes).
A weakly typed language allows many ways to bypass the type system (e.g., pointer arithmetic).
There's no fine line between them. Most languages are neither strictly strongly or weakly typed. Usually a mixture with a bias toward one or the other.
Both refer to an object in memory.
Pointers tend to make this notion more explicit:
- Dereferencing.
- Pointer arithmetic (raises issues of allocation, alignment).
- Low level operations often supported (e.g.
memcpy).
References tend to behave more like ordinary variables:
- Dereferencing still occurs, just under the hood.
- No notion of pointer arithmetic.
- Restrictions on reference variable bindings (C++).
A record consists of a set of typed fields.
Name equivalence: two types are the same if they have the same name. (e.g. Ada)
Structural equivalence: two types are equivalent if they have the same structure. (e.g. ML)
Most statically typed languages choose name equivalence. ML and Haskell are exceptions.
A variant record is a record that provides multiple alternative sets of fields, only one of which is valid at any given time. Also known as a discriminated union.
The ability to treat a class as one of its superclasses, which is the basis of OOP.
The ability to treat a value of a subtype as a value of a supertype. Related to subclass polymorphism.
The ability to treat any type uniformly. Found in ML, Haskell, and, in a very different form, in C++ templates and Java generics.
Multiple definitions of a function with the same name, each for a different set of argument types (known as overloading).
A relation between types. Similar to but not the same as subclassing.
Can be used in two different ways: subtype polymorphism and coercion.
Examples:
- 1.
{a, b, c}is a subtype of{a, c}. - 2.
a|cis a subtype ofa|b|c. - 3.Range
1..100is a subtype of1..500.
Typecasting is an explicit conversion of one type to another. Source type is known or can be inferred, destination type must be specified by the programmer.
Two variations:
- Type widening (lifting): converting subtype to supertype (coercion can also be used).
- Type narrowing: converting supertype to subtype. Involves information loss.
Subtype polymorphism: ability to treat a value of a subtype as a value of a supertype.
Coercion: ability to convert a value of a subtype to a value of a supertype.
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