E144: A mut view was taken of an immutable array

What compiler error E144 means, and how to fix it.

A slice (a[1..]) or an int[] variable doesn’t copy the array it looks at: it shares it. Writing through a mut view changes the original, which is useful:

Example

mut a: int[5] = [1, 2, 3, 4, 5];mut view: int[] = a[1..3];view[0] = 20;                      // a[1] is now 20

For an immut array that would change something you said can never change, so Turf stops you:

Example

immut a: int[3] = [1, 2, 3];mut t: int[] = a[1..];             // E144 -- 't' is a 'mut' view of 'a', which is immutablemut u = a[..];                     // E144 -- the same, with the type inferredmut w: int[] = a;                  // E144 -- a whole array is a view too

It is refused again when an existing mut view is pointed at an immutable array later:

Example

mut scratch: int[3] = [4, 5, 6];mut view: int[] = scratch[..];immut fixed: int[3] = [1, 2, 3];view = fixed[1..];                 // E144 -- 'view' is a 'mut' view of 'fixed'

To fix it, declare the view immut if you only read it (immut t = a[1..];), or make the array mut if it is meant to change. If you want to change a copy, take an editable copy with .copy(), which is a new array and not a view, so it is allowed:

Example

immut a: int[3] = [1, 2, 3];mut c = a.copy();                  // an independent int[]c[0] = 9;                          // a is untouched

Run turf --explain E144 in a terminal to read this again.