An array isn’t immutable by nature; you can’t make it constant.
The nearest you can get is:
var letter_goodness = [...]float32 {.0817, .0149, .0278, .0425, .1270, .0223, .0202, .0609, .0697, .0015, .0077, .0402, .0241, .0675, .0751, .0193, .0009, .0599, .0633, .0906, .0276, .0098, .0236, .0015, .0197, .0007 }
Note the [...] instead of []: it ensures you get a (fixed size) array instead of a slice. So the values aren’t fixed but the size is.
As pointed out by @jimt, the [...]T syntax is sugar for [123]T. It creates a fixed size array, but lets the compiler figure out how many elements are in it.