Constructor definitions take type arguments but you have given it another constructor. To fix this, you might want to create a Variable datatype and reference it in the Expresion datatype and the Statement datatype but it's hard (for me) to know without further information. On Sat, Feb 5, 2011 at 12:33 PM, Tyler Hayes wrote:

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:

Prelude> :l test.hs [1 of 1] Compiling Test              ( test.hs, interpreted )

test.hs:8:28: Not in scope: type constructor or class `Variable' Failed, modules loaded: none.

An 'assignment' is a variable followed by an expression in this example, so what can I do to fix this?

Thanks, Tyler

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