interface Variable
    exposes [ Variable, fromUtf8, toIndex, totalCount, toStr ]
    imports []

# Variables in False can only be single letters. Thus, the valid variables are "a" to "z".
# This opaque type deals with ensure we always have valid variables.
Variable : [ @Variable U8 ]

totalCount : Nat
totalCount =
    0x7A# "z"

        - 0x61# "a"

        + 1

toStr : Variable -> Str
toStr = \@Variable char ->
    when Str.fromUtf8 [ char ] is
        Ok str ->
            str

        _ ->
            "_"

fromUtf8 : U8 -> Result Variable [ InvalidVariableUtf8 ]
fromUtf8 = \char ->
    if
        char
            >= 0x61# "a"

            && char
            <= 0x7A
        # "z"
    then
        Ok (@Variable char)
    else
        Err InvalidVariableUtf8

toIndex : Variable -> Nat
toIndex = \@Variable char ->
    Num.intCast (char - 0x61)# "a"
# List.first (Str.toUtf8 "a")