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Inflation.hs

Path: build/Inflation.hs | Language: Haskell | Lines: ~237

Automatic inflation adjustment for historical dollar amounts and Bitcoin exchange rates

Overview​

Inflation.hs fights money illusion by automatically converting historical nominal dollar amounts to present-day real values. A 1950 price displayed without context is nearly meaningless—this module ensures readers see both the original amount and its inflation-adjusted equivalent.

The module handles two currencies: US dollars (using CPI/PCE data from 1913 onward) and Bitcoin (using daily USD exchange rates from 2010 onward). Dollar adjustments use year-level precision while Bitcoin requires full date precision due to its extreme volatility. Both produce rich HTML output with the adjusted amount prominent, original amount in superscript, and year in subscript.

The design philosophy prioritizes reader comprehension over precision—adjustments below 20% are suppressed as not misleading enough to warrant annotation. The module integrates at two levels: as a Pandoc AST transformer for Markdown content, and as a string-munging function for annotation titles.

Public API​

nominalToRealInflationAdjuster :: Inline -> Inline​

The primary Pandoc transformer. Accepts a Link or Span inline element containing a dollar/Bitcoin amount and converts it to an inflation-adjusted Span with metadata attributes.

Input formats:

  • Link syntax: [$50.50]($1970) or [₿0.5](₿2017-01-01)
  • Span syntax: [$50]{inflation=$2000} (for use inside link anchors)

Output: A Span with class inflation-adjusted containing:

  • The adjusted amount as primary text
  • Superscript with original amount
  • Subscript with original year
  • Data attributes for JS/CSS processing
-- Example transformation:
-- [$50.50]($1970) →
-- <span class="inflation-adjusted"
--       data-year-original="1970" data-amount-original="50.50"
--       data-year-current="2024" data-amount-current="343.83"
--       title="CPI inflation-adjusted...">
--   $343.83<span class="subsup"><sup>$50.50</sup><sub>1970</sub></span>
-- </span>

Called by: hakyll.hs (page compilation), LinkMetadata.hs (annotation processing) Calls: dollarAdjuster, bitcoinAdjuster

nominalToRealInflationAdjusterHTML :: String -> String -> String​

String-based inflation adjustment for HTML annotation titles. Takes a date string and HTML content, finds dollar amounts, and wraps them in inflation spans.

nominalToRealInflationAdjusterHTML :: String  -- ^ Date (YYYY-MM-DD format)
                                   -> String  -- ^ HTML content with $ amounts
                                   -> String  -- ^ HTML with adjusted amounts

Called by: LinkMetadata.hs:439 (title processing) Calls: nominalToRealInflationAdjuster, Utils.toHTML

Important: Must be called after typesetHtmlField/titlecase' as those transforms may break the inline HTML spans.

inflationDollarTestSuite :: [((Text, Text), Inline, Inline)]​

Returns failing test cases (expected ≠ actual). Used by Test.hs for regression testing. Returns empty list when all tests pass.

Called by: Test.hs:24

isInflationURL :: Text -> Bool​

Checks if a URL is an inflation pseudo-URL (starts with $ or ₿).

Note: Defined in Utils.hs but re-exported from Inflation to avoid circular dependencies.

Called by: Interwiki.hs, Query.hs, generateLinkBibliography.hs, Utils.host

isInflationLink :: Inline -> Bool​

Checks if a Pandoc Inline is an inflation-annotated link or span.

Called by: Various link processing functions

Internal Architecture​

Core Adjustment Flow​

nominalToRealInflationAdjuster
    ├── dollarAdjuster (for $YYYY URLs)
    │   ├── multiplyByUnits (handles k/m/b/t suffixes)
    │   ├── dollarAdjust
    │   │   └── inflationAdjustUSD (applies cumulative inflation rates)
    │   └── printDouble (formatting)
    │
    └── bitcoinAdjuster (for ₿YYYY-MM-DD URLs)
        ├── bitcoinAdjust
        │   └── bitcoinQuery (daily exchange rate lookup)
        ├── bitcoinUSDExchangeRate (inflation-adjusted rate map)
        └── dollarAdjust (adjusts historical USD to present)

Data Structures​

Inflation rates (Config.Inflation.inflationRatesUSD): List of annual percentage changes indexed by (year - 1913). CPI data for 1913-1958, PCE data from 1959 onward. The last rate is repeated infinitely for future projections.

Bitcoin rates (Config.Inflation.bitcoinUSDExchangeRateHistory): Association list of (YYYY-MM-DD, rate) pairs. Missing dates are linearly interpolated; dates outside range carry forward/backward.

Output Span Structure​

<span class="inflation-adjusted"
      data-year-original="1970"
      data-amount-original="50.50"
      data-year-current="2024"
      data-amount-current="343.83"
      title="CPI inflation-adjusted US dollar: from nominal $50.50 in 1970 → real $343.83 in 2024">
  $343.83
  <span class="subsup">
    <sup>$50.50</sup>
    <sub>1970</sub>
  </span>
</span>

Key Patterns​

Minimum Threshold Suppression​

Small adjustments (<20%) are stripped of annotation rather than shown, reducing noise for recent amounts:

if (adjustedDollar / oldDollar) < C.minPercentage || oldDollar == 0
   then Span nullAttr text  -- just strip the annotation
   else ...  -- full inflation span

This means [$100]($2023) in 2024 produces just $100 with no adjustment markup.

Unit Multiplier Expansion​

The multiplyByUnits function expands shorthand notation:

-- '$50.5k' → 50500.0
-- '$1.2m'  → 1200000.0
-- '$3.5b'  → 3500000000.0
-- '$1t'    → 1000000000000.0

Cumulative Inflation Calculation​

Inflation is computed by multiplying annual rates between the two years:

inflationAdjustUSD :: Double -> Int -> Int -> Double
inflationAdjustUSD d yOld yCurrent = d * product rates
  where
    percents = slice (yOld-1913) (yCurrent-1913) C.inflationRatesUSD
    rates = map (\r -> 1 + (r/100)) percents

Bitcoin Double-Adjustment​

Bitcoin amounts undergo two adjustments:

  1. Convert historical BTC to historical USD at that day's exchange rate
  2. Inflation-adjust that USD amount to present day
bitcoinAdjust cy oldBitcoinAmount oldDate =
  let oldExchangeRate = bitcoinQuery cy oldDate
  in oldBitcoinAmount * oldExchangeRate  -- then inflationAdjustUSD is applied

Configuration​

Config/Inflation.hs (~1,098 lines)​

ExportTypeDescription
minPercentageDouble1.20 — suppress adjustments below 20%
inflationRatesUSD[Double]Annual % changes, 1913-present (extended with repeat)
bitcoinUSDExchangeRateHistory[(String, Double)]Daily rates from 2010-05-20 onward
inflationDollarLinkTestCases[((Text,Text), Inline)]Regression test cases

Config/Misc.hs​

ExportTypeDescription
currentYearIntRuntime-computed current year (via unsafePerformIO)

Updating Inflation Data​

To update rates:

  1. CPI/PCE rates: Add new annual values to pce20192023 (or create a new chunk) in Config/Inflation.hs
  2. Bitcoin rates: Append new daily entries to bitcoinUSDExchangeRateHistory
  3. Test cases: Update expected values in inflationDollarLinkTestCases for the new currentYear

The last available rate is carried forward indefinitely, so stale data degrades gracefully.

Integration Points​

Pandoc Pipeline Integration​

Called in two places during page compilation:

-- hakyll.hs:365 — page body processing
walk (map nominalToRealInflationAdjuster) pb

-- LinkMetadata.hs:447 — annotation body processing
walk (linkIcon . linkLive . nominalToRealInflationAdjuster) $

Annotation Title Processing​

-- LinkMetadata.hs:439
let titleHtml = nominalToRealInflationAdjusterHTML c $ typesetHtmlField $ titlecase' a

URL Filtering​

isInflationURL is used throughout the codebase to identify and skip inflation pseudo-URLs that shouldn't be treated as real links:

  • Utils.host: Skip extracting host from inflation URLs
  • Interwiki.hs: Distinguish from interwiki links
  • Query.hs, generateLinkBibliography.hs: Filter out during link analysis

CSS Classes​

The frontend can style inflation adjustments via:

  • .inflation-adjusted — the outer container
  • .subsup — the superscript/subscript wrapper

Data attributes enable JS-based dynamic updates or tooltips.

See Also​