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fixed length arrays as types
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Date: 2000-11-06 (20:06)
From: Chris Hecker <checker@d...>
Subject: fixed length arrays as types

Is there any way to do this:

type vector3 = [| float; float; float |];

Basically, I want an array of a given length to be a given type, so I can use the type system to check add_vector3 rather than throwing if the arrays don't match.  I know I can make records {x:float, y:float} but I'd like it to be parameterizable at compile time.

Something like this C++:

template <int unsigned N> class vector { float a[N]; };
vector<3> add( vector<3> v1, vector<3> v2 );

vector<3> v3 = add(vector<3>(),vector<3>());    // works
vector<3> v4 = add(vector<5>(),vector<3>());    // type error (note v<5>

I guess the higher level question is whether scalar constants can be part of the type signature like they can be in C++.  Or, the related but different question is whether there's a way to differentiate between "float a[]" (or "float *a"), the variable length array type, and "float a[3]", a fixed length array type, which C++ doesn't do, but it lets you wrap the ideas in template classes which do allow you to represent this to the type system.