Question

Is there any way to get static checks on Haskell arrays? Let’s take this code:

import Data.Array
let a = listArray (0, 10) [-3.969683028665376e+01, 2.209460984245205e+02, -2.759285104469687e+02, 1.383577518672690e+02, -3.066479806614716e+01, 2.506628277459239e+00]

(0, 10) should really be (0, 5), but the compiler accepts the code. The error is only detected at runtime, despite the fact that it could be detected at compile-time.

Answer 1

This cannot be detected at compile-time, because there’s nothing in the list’s type that saves its size, so the listArray function cannot possibly perform such checks. Also, if the data came from an external file (for example), it would be very difficult to get the static size checking to work.

You need a dependent type system such as the one you find in Agda for things like this to be possible.