On Fri, Feb 4, 2011 at 10:33 PM, Tyler Hayes
As an exercise, I'm attempting to create an abstract syntax tree for a very small language:
data Expression = Variable String | Integer | Expression BinOp Expression deriving Show
data Statement = Assignment Variable Expression --error! | If Expression Statement Statement | While Expression Statement | Compound Statement deriving Show
data BinOp = Add | Mult | LessThan | GreaterThan deriving Show
However, there is a problem with having "Variable" as an argument to the "Assignment" constructor of the "Statement" data type:
In your example, you've declared three types: Expression, Statement and BinOp. So when you ask for a type named 'Variable' it can't find one. Perhaps you meant 'String' ? In your above example, you do have a data constrictor 'Variable', which can be used as a value to construct values of type 'Expression' (or in a pattern match over values of type Expression). The difference between types and data constructors can be tricky at first, especially since they appear with each other in code and have similar naming conventions. If I'm speaking a foreign language I can probably dig up a tutorial to point you to :-) Antoine