LinkLive.hs

Path: build/LinkLive.hs | Language: Haskell | Lines: ~156

Determines which external URLs can be shown as live iframe previews based on domain whitelists.

Overview

LinkLive.hs decides at compile-time which external links can be previewed as live iframes (rather than requiring annotations or forcing full page loads). When a URL passes the whitelist checks, the module adds the CSS class link-live to the link. The frontend JavaScript (per the module comment, extracts-contents.js; implemented in content.js) reads this class to enable iframe-based popups.

The challenge is that most external websites cannot be iframed: they set X-Frame-Options headers, use JavaScript that breaks in frames, have mixed HTTP/HTTPS content, or have unusable layouts (giant sticky headers, etc.). Gwern estimates only about 25% of external links work as live popups. Rather than disappointing readers with broken previews, the site whitelists domains that have been manually verified to work.

The configuration lives in Config/LinkLive.hs - a 4,600-line file containing domain lists organized by:

goodDomainsSimple: exact domain matches (~700 domains)
goodDomainsSub: suffix patterns like .github.io (~18 patterns)
badDomainsSimple: known-broken exact domains (~2,300 domains)
badDomainsSub: known-broken suffix patterns (~7 patterns)
goodLinks/badLinks: specific URLs for test cases

Public API

`linkLive :: Inline -> Inline`

Main entry point. Adds link-live class to links that pass the whitelist.

linkLive (Link (_, cl, kvs) _ (u, _))
  | "link-live-not" `elem` cl             = x           -- explicit opt-out
  | "data-url-archive" `elem` map fst kvs = addClass x  -- local archives always work
  | linkLiveP u                           = addClass x  -- whitelist check
  | otherwise                             = x

Called by: Typography.hs (in typographyTransformPermanent) Calls: linkLiveP, addClass

`linkLiveP :: Text -> Bool`

Predicate: can this URL be shown as a live iframe?

linkLiveP u
  | u `elem` overrideLinkLive     = True   -- explicit override
  | u `elem` overrideLinkLiveNot  = False  -- explicit block
  | "http://" `isPrefixOf` u      = False  -- no HTTP (mixed content)
  | "/" `isPrefixOf` u            = False  -- local links don't match
  | otherwise                     = urlLive u == Just True

Called by: linkLive, external callers checking liveness

`urlLive :: Text -> Maybe Bool`

Core lookup logic. Returns Just True (known good), Just False (known bad), or Nothing (unknown/untested).

urlLive u
  | host u `elem` badDomainsSimple  = Just False
  | anySuffixT (host u) badDomainsSub  = Just False
  | host u `elem` goodDomainsSimple = Just True
  | anySuffixT (host u) goodDomainsSub = Just True
  | anyInfixT u wikipediaURLs       = Just $ wikipediaLiveP u
  | otherwise                       = miscUrlRules u

Precedence: bad simple > bad sub > good simple > good sub > Wikipedia > misc rules

`wikipediaLiveP :: Text -> Bool`

Special handling for Wikipedia. Most Wikipedia pages work as live popups except Special:* namespace (sets X-Frame headers for security).

wikipediaLiveP u = "link-live" `elem` wpPopupClasses u

Delegates to Interwiki.wpPopupClasses which parses the URL path and checks against linkliveNamespacesNo = ["Special", "/w/index.php"].

`linkLivePrioritize :: IO [(Int, Text)]`

Build-time helper: finds untested domains with high link counts. Appends test cases to /lorem-link.md for manual review.

linkLivePrioritize = do
  backlinks <- readBacklinksDB
  -- Group by domain, filter untested, sort by count
  -- Write high-frequency domains to test page

Called by: sync.sh during build Writes to: lorem-link.md

`linkLiveTest :: [(Text, Bool)]`

Unit test suite. Returns URLs that fail their expected classification (good URL not getting link-live, bad URL incorrectly getting it).

Called by: Test.hs

`linkLiveTestHeaders :: IO ()`

Runtime header check. Uses curl to fetch headers for all goodLinks and warns if X-Frame-Options or frame-ancestors is detected.

linkLiveTestHeaders = forM_ goodLinks $ \u -> do
  (_, _, headers) <- runShellCommand "curl" ["--head", u]
  when ("x-frame" `isInfixOf` headers) $
    printRed u >> print "X-FRAME option detected"

Internal Architecture

Decision Flow

URL input
    │
    ├─ overrideLinkLive?     → True
    ├─ overrideLinkLiveNot?  → False
    ├─ http:// prefix?       → False (mixed content blocked by browsers)
    ├─ local path?           → False
    │
    └─ urlLive lookup:
        ├─ badDomainsSimple?   → False
        ├─ badDomainsSub?      → False
        ├─ goodDomainsSimple?  → True
        ├─ goodDomainsSub?     → True
        ├─ Wikipedia?          → wikipediaLiveP check
        └─ miscUrlRules        → special cases (YouTube embeds, Markdeep)

Config/LinkLive.hs Structure

-- Override lists (highest priority)
overrideLinkLive, overrideLinkLiveNot :: [Text]

-- Domain patterns
goodDomainsSub    :: [Text]  -- e.g., [".github.io", ".wordpress.com"]
goodDomainsSimple :: [Text]  -- e.g., ["gwern.net", "lesswrong.com"]
badDomainsSub     :: [Text]  -- e.g., [".substack.com", ".medium.com"]
badDomainsSimple  :: [Text]  -- e.g., ["arxiv.org", "twitter.com"]

-- Test URLs with known status
goodLinks, badLinks :: [Text]

-- Special-case rules
miscUrlRules :: Text -> Maybe Bool
miscUrlRules u
  | "youtube.com/embed/" `isPrefixOf` u = Just True  -- embeds work
  | "youtube.com/" `isPrefixOf` u       = Just False -- regular YT blocked
  | "casual-effects.com" `isPrefixOf` u = Just $ not $ ".md.html" `isInfixOf` u
  | otherwise = Nothing

Key Patterns

Why Whitelist Instead of Blacklist?

Only ~25% of external sites work in iframes. A blacklist would require maintaining thousands of entries and would still miss new broken sites. The whitelist approach means:

New domains default to "unknown" (no live popup offered)
Manual testing adds verified domains
linkLivePrioritize surfaces high-frequency untested domains

HTTP vs HTTPS

HTTP links are unconditionally rejected:

| "http://" `T.isPrefixOf` u = False

Browsers block "mixed content" - an HTTPS page (gwern.net) cannot load HTTP iframes. Even if the HTTP version of a site works, it's blocked by browser security.

Local Archive Fallback

Links with data-url-archive always get link-live:

| "data-url-archive" `elem` map fst kvs = addClass x

If gwern.net has a local mirror of the page (via LinkArchive.hs), the popup can use that instead of the live external site.

Configuration

Setting	Location	Effect
`testPage`	Config/LinkLive.hs	Where `linkLivePrioritize` appends test cases
`linkLivePrioritizeMinimum`	Config/LinkLive.hs	Minimum backlink count to surface for testing (default: 3)
`linkLivePrioritizeBlacklist`	Config/LinkLive.hs	Domains to never suggest for testing

Integration Points

Build-Time Integration

Typography.hs calls linkLive during permanent transforms
Test.hs runs linkLiveTest to catch regressions
sync.sh may call linkLivePrioritize to surface new domains

Runtime Integration (Frontend)

extracts-contents.js (implemented in content.js) checks for link-live class:

// content.js:465
&& link.classList?.contains("link-live") == true

When detected, the popup system creates an iframe instead of fetching/transforming content.

link-live: URL can be iframed
link-live-not: Explicit opt-out (author override)
content-transform: Use API-based content (Wikipedia, GitHub)
content-transform-not: Skip content transforms

Overview​

Public API​

linkLive :: Inline -> Inline​

linkLiveP :: Text -> Bool​

urlLive :: Text -> Maybe Bool​

wikipediaLiveP :: Text -> Bool​

linkLivePrioritize :: IO [(Int, Text)]​

linkLiveTest :: [(Text, Bool)]​

linkLiveTestHeaders :: IO ()​

Internal Architecture​

Decision Flow​

Config/LinkLive.hs Structure​

Key Patterns​

Why Whitelist Instead of Blacklist?​

HTTP vs HTTPS​

Local Archive Fallback​

Configuration​

Integration Points​

Build-Time Integration​

Runtime Integration (Frontend)​

Related Classes​

See Also​