diff --git a/Makefile b/Makefile
index 3689efd..da29f56 100644
--- a/Makefile
+++ b/Makefile
@@ -55,7 +55,7 @@ define copy
   cp $< $@
 endef
 
-$(LANGFILTER): langfilter/*.hs langfilter/langfilter.cabal
+$(LANGFILTER): langfilter/lib/*.hs langfilter/main/*.hs langfilter/langfilter.cabal
 	$(cabal-exe)
 
 $(LAANTAS_SCRIPT): laantas-script/*
diff --git a/langfilter/langfilter.cabal b/langfilter/langfilter.cabal
index bde2dee..1ac2656 100644
--- a/langfilter/langfilter.cabal
+++ b/langfilter/langfilter.cabal
@@ -7,15 +7,7 @@ license: AGPL-3.0-or-later
 author: Rhiannon Morris <rhi@rhiannon.website>
 maintainer: Rhiannon Morris <rhi@rhiannon.website>
 
-executable langfilter
-  hs-source-dirs: .
-  main-is: Main.hs
-  other-modules:
-    Lang,
-    Ebnf,
-    Spans,
-    LaantasImage,
-    Glosses
+common base
   ghc-options:
     -Wall -Wno-missing-signatures -Wno-missing-pattern-synonym-signatures
     -Wno-name-shadowing
@@ -31,6 +23,7 @@ executable langfilter
     OverloadedStrings,
     PatternSynonyms,
     RecordWildCards,
+    TupleSections,
     ViewPatterns
   build-depends:
     base >= 4.14.0.0 && < 4.21,
@@ -42,3 +35,20 @@ executable langfilter
     text ^>= 2.1,
     pretty-show ^>= 1.10,
     laantas-script
+
+library
+  import: base
+  hs-source-dirs: lib
+  exposed-modules:
+    Lang,
+    Ebnf,
+    Spans,
+    LaantasImage,
+    Glosses,
+    LangFilter
+
+executable langfilter
+  import: base
+  hs-source-dirs: main
+  main-is: Main.hs
+  build-depends: langfilter
diff --git a/langfilter/Ebnf.hs b/langfilter/lib/Ebnf.hs
similarity index 100%
rename from langfilter/Ebnf.hs
rename to langfilter/lib/Ebnf.hs
diff --git a/langfilter/Glosses.hs b/langfilter/lib/Glosses.hs
similarity index 100%
rename from langfilter/Glosses.hs
rename to langfilter/lib/Glosses.hs
diff --git a/langfilter/LaantasImage.hs b/langfilter/lib/LaantasImage.hs
similarity index 100%
rename from langfilter/LaantasImage.hs
rename to langfilter/lib/LaantasImage.hs
diff --git a/langfilter/Lang.hs b/langfilter/lib/Lang.hs
similarity index 100%
rename from langfilter/Lang.hs
rename to langfilter/lib/Lang.hs
diff --git a/langfilter/Main.hs b/langfilter/lib/LangFilter.hs
similarity index 84%
rename from langfilter/Main.hs
rename to langfilter/lib/LangFilter.hs
index f4ae468..b411594 100644
--- a/langfilter/Main.hs
+++ b/langfilter/lib/LangFilter.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE TupleSections #-}
+module LangFilter where
 
 import Lang
 import Ebnf
@@ -6,7 +6,6 @@ import Spans
 import Glosses
 
 import Text.Pandoc.Definition
-import Text.Pandoc.JSON
 import Text.Pandoc.Walk
 import Data.Maybe
 import Data.Map (Map)
@@ -18,17 +17,16 @@ import Control.Monad
 import System.Environment
 
 
-main :: IO ()
-main = toJSONFilter filter where
-  filter p@(Pandoc (Meta m) _) = do
-    lang <- toLang $ Map.lookup "conlang" m
-    defColor <- getDefColor m
-    let ?lang = lang
-    let ?defColor = defColor
-    let f = map (walk spans . fixFigureClass . makeEbnf .
-                 makeQuotes . letterList) .
-            concatMap (makeBlocks <=< glosses)
-    pure $ walk f p
+langFilter :: Pandoc -> IO Pandoc
+langFilter p@(Pandoc (Meta m) _) = do
+  lang <- toLang $ Map.lookup "conlang" m
+  defColor <- getDefColor m
+  let ?lang = lang
+  let ?defColor = defColor
+  let f = map (walk spans . fixFigureClass . makeEbnf .
+               makeQuotes . letterList) .
+          concatMap (makeBlocks <=< glosses)
+  pure $ walk f p
 
 getDefColor :: Map Text MetaValue -> IO Text
 getDefColor m = do
diff --git a/langfilter/Spans.hs b/langfilter/lib/Spans.hs
similarity index 100%
rename from langfilter/Spans.hs
rename to langfilter/lib/Spans.hs
diff --git a/langfilter/main/Main.hs b/langfilter/main/Main.hs
new file mode 100644
index 0000000..549f38b
--- /dev/null
+++ b/langfilter/main/Main.hs
@@ -0,0 +1,4 @@
+import LangFilter
+import Text.Pandoc.JSON
+
+main = toJSONFilter langFilter