use mtl to generate errors

1 files changed, 31 insertions, 0 deletions

diff --git a/src/Error.hs b/src/Error.hs
new file mode 100644
index 0000000..276d3e2
--- /dev/null
+++ b/src/Error.hs
@@ -0,0 +1,31 @@
+module Error (
+             LispError (..)
+             , LispResult (..)
+             , unwrap
+             ) where
+
+import           Control.Monad.Except
+import           Parser
+import           Text.ParserCombinators.Parsec
+
+data LispError = Parse ParseError
+               | BadForm String Expr
+               | ArgCount Int [Expr]
+               | UnknownFunction String
+               | TypeMismatch String Expr
+
+unwordsList :: [Expr] -> String
+unwordsList = unwords . map show
+
+instance Show LispError where
+    show (Parse e)        = "Parser Error: " ++ show e
+    show (BadForm s expr) = "Bad Form: " ++ s ++ ": " ++ show expr
+    show (ArgCount n es) = "Invalid arity, expected " ++ show n ++ ", got values: " ++ unwordsList es
+    show (UnknownFunction fn) = "Unknown reference to function: " ++ fn
+    show (TypeMismatch msg got) = "Type mismatch, expected " ++ msg ++ ", got: " ++ show got
+
+type LispResult = Either LispError
+
+unwrap :: LispResult t -> t
+unwrap (Right v) = v
+unwrap (Left _)  = undefined -- should panic