test(tests/playground/eval): proof of concept for a safe eval function

tests/playground/eval.lean

@@ -0,0 +1,35 @@
+import init.lean.name
+open Lean
+
+abbrev ConstantTable (α : Type) := HashMap Name α
+
+def mkSimpleFnTable : IO (IO.Ref (ConstantTable (Nat → Nat))) :=
+IO.mkRef {}
+
+@[init mkSimpleFnTable]
+constant simpleFnTable : IO.Ref (ConstantTable (Nat → Nat)) := default _
+
+def registerSimpleFn (n : Name) (fn : Nat → Nat) : IO Unit :=
+do ini ← IO.initializing,
+   unless ini (throw "we should only register functions during initialization"),
+   simpleFnTable.modify (λ m, m.insert n fn)
+
+def add10 (n : Nat) := n+10
+def mul10 (n : Nat) := n*10
+def inc (n : Nat) := n+1
+
+@[init] def regAdd10 : IO Unit := registerSimpleFn `add10 add10
+@[init] def regMul10 : IO Unit := registerSimpleFn `mul10 mul10
+@[init] def regInc   : IO Unit := registerSimpleFn `inc inc
+
+def eval (fName : Name) (n : Nat) : IO Nat :=
+do m ← simpleFnTable.get,
+   match m.find fName with
+   | some f := pure $ f n
+   | none   := throw (IO.userError "unknow function")
+
+def main (xs : List String) : IO Unit :=
+do [f, x] ← pure xs | throw "invalid number of arguments",
+   r ← eval (mkSimpleName f) x.toNat,
+   IO.println r,
+   pure ()