definition module sem

from Data.Either import :: Either
from AST import :: AST, :: Pos, :: Type
from StdOverloaded import class toString

:: SemError 
	= ParseError Pos String 
	| UnifyError Pos Type Type 
	| UnifyErrorStub Type Type 
	| Error String
:: SemOutput :== Either [SemError] AST


instance toString SemError

sem :: AST -> SemOutput