Add Handle Bad Numerical Amounts With BigDecimal as a Rails TIL

rails/handle-bad-numerical-amounts-with-big-decimal.md

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+# Handle Bad Numerical Amounts With BigDecimal
+
+I'm working on a payment page with a backing Rails controller. The user can
+select between their full balance or some partial payment amount. Because this
+form accepts an arbitrary value for the amount, I need to do some server-side
+validation.
+
+While I could parse the `amount` value and handle the exception that gets raised
+on bad numerical values, I'd rather have bad values coerce to `nil` and let
+downstream validations handle it from there.
+
+[`BigDecimal`](https://docs.ruby-lang.org/en/master/BigDecimal.html) can help
+here with the support of its `exception` option.
+
+```ruby
+> BigDecimal('123', exception: false)
+=> 0.123e3
+> BigDecimal('taco', exception: false)
+=> nil
+```
+
+Maybe that parsing logic ends up looking something like this:
+
+```ruby
+def parse_payment_amount(value, current_balance)
+  amount = BigDecimal(value, exception: false)
+
+  if amount.present? && amount == current_balance
+    [:full_balance, amount]
+  else
+    [:other, amount]
+  end
+end
+```
+
+I return tuples where you either get a verified `:full_balance` amount or you
+get some `:other` amount. The other amount could be `nil` which would trigger a
+downstream validation.