libghc-decimal-dev binary package in Ubuntu Focal amd64

 Library for manipulating decimal numebers, primary for financial arithmetic.
 .
 Decimal number has an integer mantissa and a negative
 exponent. The exponent can be interpreted as the number
 of decimal places in the value.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.

Source package