Suppose I have a type describing a statement and that I'm trying to make it an instance of arbitrary. The type looks like this: data Statement = InputDecs [InputDecl] | VarDecs [VarDecl] | ArrayDecs [ArrayDecl] | Compound [Statement] | Assign (VarIdent, Expr) | ArrayAssign (VarIdent, [Expr], Expr) Assuming that other types involved were instances of arbitrary, how do I write arbitrary for Statement? Poking around various bits of source code I think that for a type like the following data Style = StyleValue Expr | Solid | Dashed | Dotted | Dashed2 | Dashed3 deriving Show I can write instance Arbitrary Style where arbitrary = oneOf [ StyleValue . arbitrary, elements [ Solid , Dashed , Dotted , Dashed2 , Dashed3 ] ] I'm not sure if this is correct, though, so any help is appreciated! Thanks in advance, Joel -- http://wagerlabs.com/