33 lines
2.0 KiB
Plaintext
33 lines
2.0 KiB
Plaintext
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Chapel’s type system can be surprisingly powerful. In addition to “usual”
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features such as generics and polymorphism, Chapel provides the ability to
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manipulate types using functions; this involves both taking types as arguments
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to functions and returning them from these functions. This can enable powerful
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programming techniques that are typically confined to the domain of
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metaprogramming.
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For example, although Chapel’s notion of compile-time values — ‘param’s — is
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limited to primitive types such as integers, booleans, and strings, one can
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encode compile-time lists of these values as types. Such encodings can be used
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to create compile-time specializations of functions that would be otherwise
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tedious to write by hand. One use case for such specializations is the
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implementation of a family of functions for approximating differential
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equations, the Adams-Bashforth methods. Some variants of these methods can be
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encoded as lists of coefficients. Thus, it becomes possible to define a single
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function that accepts a type-level list of coefficients and produces a
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“stamped out” implementation of the corresponding method. This reduces the
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need to implement each method explicitly by hand. Another use case of function
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specialization is a type-safe ‘printf’ function that validates that users’
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format specifiers match the type of arguments to the function.
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More generally, Chapel’s types can be used to encode algebraic sums (disjoint
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unions) and products (Cartesian) of types. This, in turn, makes it possible to
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build arbitrary data structures at the type level. The lists-of-values case
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above is an instance of this general principle. Functions can be defined on
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type-level data structures by relying on overloading and type arguments.
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Programming in this manner starts to resemble programming in a purely
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functional language such as Haskell.
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Though this style of programming has not seen much use thus far, it can be a
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powerful technique for controlling types of arguments or constructing highly
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customized functions with no runtime overhead.
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