move Error to its own file

src/Lib/Error.newt

@@ -0,0 +1,37 @@
+module Lib.Error
+
+import Prelude
+import Lib.Common
+
+-- I'll want to get Context / Val in some of these
+-- and a pretty printer in the monad
+data Error
+  = E FC String
+  | ENotFound FC String
+  | Postpone FC QName String
+
+
+instance HasFC Error where
+  getFC (E x str) = x
+  getFC (ENotFound x _) = x
+  getFC (Postpone x k str) = x
+
+errorMsg : Error -> String
+errorMsg (E x str) = str
+errorMsg (ENotFound x nm) = "\{nm} not in scope"
+errorMsg (Postpone x k str) = str
+
+showError : (src : String) -> Error -> String
+showError src err =
+  let fc = getFC err
+  in "ERROR at \{show $ getFC err}: \{errorMsg err}\n" ++ go fc 0 (lines src)
+  where
+    go : FC → Int → List String → String
+    go fc l Nil = ""
+    go fc l (x :: xs) =
+      if l == fcLine fc then
+        let width = fc.bnds.endCol - fc.bnds.startCol in
+        "  \{x}\n  \{replicate (cast $ fcCol fc) ' '}\{replicate (cast width) '^'}\n"
+      else if fcLine fc - 3 < l then "  " ++ x ++ "\n" ++ go fc (l + 1) xs
+      else go fc (l + 1) xs
+