diff --git a/language-ats.cabal b/language-ats.cabal
--- a/language-ats.cabal
+++ b/language-ats.cabal
@@ -1,5 +1,5 @@
 name:                language-ats
-version:             0.1.0.1
+version:             0.1.0.3
 synopsis:            Parser and pretty-printer for ATS.
 description:         Parser and pretty-printer for [ATS](http://www.ats-lang.org/), written with Happy and Alex.
 homepage:            https://github.com/vmchale/language-ats#readme
@@ -36,6 +36,7 @@
                      , lens
                      , deepseq
                      , ansi-wl-pprint >= 0.6.8
+                     , composition-prelude >= 0.1.1.2
                      , recursion-schemes
                      , ansi-terminal
                      , unordered-containers
diff --git a/src/Language/ATS/Parser.y b/src/Language/ATS/Parser.y
--- a/src/Language/ATS/Parser.y
+++ b/src/Language/ATS/Parser.y
@@ -423,6 +423,7 @@
               | let ATS in Expression extern {% Left $ Expected $5 "end" "extern" }
               | let ATS in Expression fun {% Left $ Expected $5 "end" "fun" }
               | let ATS in Expression vtypedef {% Left $ Expected $5 "end" "vtypedef" }
+              | let ATS in Expression implement {% Left $ Expected $5 "end" "implement" }
               | if Expression then Expression else else {% Left $ Expected $6 "Expression" "else" }
 
 -- | Parse a termetric
diff --git a/stack.yaml b/stack.yaml
--- a/stack.yaml
+++ b/stack.yaml
@@ -2,7 +2,8 @@
 resolver: lts-10.3
 packages:
   - '.'
-extra-deps: []
+extra-deps:
+  - composition-prelude-1.1.0.0
 flags:
   language-ats:
     development: false