Problem Statement

Create a function that takes a Roman numeral as its argument and returns its value as a numeric decimal integer. You don't need to validate the form of the Roman numeral. Modern Roman numerals are written by expressing each decimal digit of the number to be encoded separately, starting with the leftmost decimal digit and skipping any 0s   (zeroes). 1990 is rendered as   MCMXC     (1000 = M,   900 = CM,   90 = XC)     and 2008 is rendered as   MMVIII       (2000 = MM,   8 = VIII). The Roman numeral for 1666,   MDCLXVI,   uses each letter in descending order.

Let's start with the solution:

⟨ToArabic⇐A⟩ ← {
  c ← "IVXLCDM"         # Characters
  v ← ⥊ (10⋆↕4) ×⌜ 1‿5  # Their values
  A ⇐ +´∘(⊢ׯ1⋆<⟜«) v ⊏˜ c ⊐ ⊢
}


   ToArabic¨ "MCMXC"‿"MDCLXVI"‿"MMVII"‿"MMXXI"
⟨ 1990 1666 2007 2021 ⟩