Fixed-length arrays in ALFE

This is part of the ALFE types series.

Foo[2], Foo[3], Foo[4] etc. are arrays whose length is fixed at compile time (not to be confused with [Foo] whose length is unknown and possibly undefined even at runtime). Passing around a value of array type copies the entire array (arrays don't decay to pointers). Operations defined on the elements can also be applied (element-wise) to the array itself, so you can write things like:

Int[2] centre(Int[2] topLeft, Int[2] bottomRight)
    return (topLeft + bottomRight)/2;

Foo[2] also has a template form, but since template arguments can only be types and not integers like they can in C++, there is a twist: Array<Foo, Class {Int n=2;}>. Any type can be used for the second argument as long as it has a public member named n of type Int with value known at compile time. This allows templates to work generically with arrays of different lengths.

Foo[n] can be indexed by an integer to yield an (RValue or LValue) Foo. It can also be indexed by a sequence (whose values are known at run time) to yield a slice of the array.

An array can be coerced to a sequence, and the compiler will box up the element count and pointer as necessary.

One Response to “Fixed-length arrays in ALFE”

  1. […] In C++ there are two kinds of things you can use as template arguments. One is the obvious one, types, but less well known is that integers (and some other values) can also be used. Early on in the design of ALFE's type system I decided to simplify things by having template arguments all be type constructors. If you really need to use an integer (or indeed any other value) as a template argument you can wrap it in a type. […]

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