reintroduce the compiler API

src/compiler/api/GF/Command/TreeOperations.hs

@@ -0,0 +1,39 @@
+module GF.Command.TreeOperations (
+  treeOp,
+  allTreeOps,
+  ) where
+
+import PGF2(Expr,PGF,Fun,compute,mkApp,unApp,unMeta,exprSize,exprFunctions)
+import Data.List
+
+type TreeOp = [Expr] -> [Expr]
+
+treeOp :: PGF -> String -> Maybe (Either TreeOp (Fun -> TreeOp))
+treeOp pgf f = fmap snd $ lookup f $ allTreeOps pgf
+
+allTreeOps :: PGF -> [(String,(String,Either TreeOp (Fun -> TreeOp)))]
+allTreeOps pgf = [
+   ("compute",("compute by using semantic definitions (def)",
+      Left  $ map (compute pgf))),
+   ("largest",("sort trees from largest to smallest, in number of nodes",
+      Left  $ largest)),
+   ("nub",("remove duplicate trees",
+      Left  $ nub)),
+   ("smallest",("sort trees from smallest to largest, in number of nodes",
+      Left  $ smallest)),
+   ("subtrees",("return all fully applied subtrees (stopping at abstractions), by default sorted from the largest",
+      Left  $ concatMap subtrees)),
+   ("funs",("return all fun functions appearing in the tree, with duplications",
+      Left  $ \es -> [mkApp f [] | e <- es, f <- exprFunctions e]))
+  ]
+
+largest :: [Expr] -> [Expr]
+largest = reverse . smallest
+
+smallest :: [Expr] -> [Expr]
+smallest = sortBy (\t u -> compare (exprSize t) (exprSize u))
+
+subtrees :: Expr -> [Expr]
+subtrees t = t : case unApp t of
+  Just (f,ts) -> concatMap subtrees ts
+  _ -> []  -- don't go under abstractions