Question

I’ve already asked the question about undefined. And because it is impossible to do such thing, I will describe the case when this can be useful. But I think the question is common and can be useful in other cases.

Lets imagine a situation, when we write a parser for some grammar. We’ve defined AST implemented by many data-types, each of them has a set of constructors, each constructor has a set of parameters. Every type derives from Show. Then we begin to write parser realization, where we define many functions with recursive dependences.

As we have many functions, its hard to write a whole module at once without a check. But functions circular reference won’t allow to compile the unfinished module. In this case I define hook functions that do nothing expect returning value of the type needed.

And here is a problem! Function must return not a simple type, but a data constructor with constructors in constructors. This is too hard for a hook function. So I do this:

parseHook _ = undefined

And everything is simple till “undefined” is not in the result data tree. I want to point out that this situation happens in developing stage and that the only thing I want is to see the tree structure. Of course, I can wrap every type in “Maybe”, but I dont want to change already existing types.

English is not my native language, so Im not sure that I’ve expressed my thoughts correctly. That’s why I want to add an example of this situation:

data StructParam = StructParam Int Int
                 deriving Show 

data Struct1 = Struct1 Int Int StructParam Int Int Int Int StructParam
             deriving Show 

data Struct2 = Struct2 Int Int StructParam Int Int Int Int StructParam  
             deriving Show 

data Struct3 = Struct3_var1 Struct1 
             | Struct3_var2 Struct2
             deriving Show 

-- this hook is ok, but too long
parse1 _ = Struct1 1 2 (StructParam 3 4) 5 6 7 8 (StructParam 9 0)

-- this hook is short, but undefined
parse2 _ = undefined

-- complete function
rootParse 1 = Struct3_var1(parse1 1)    
rootParse 2 = Struct3_var2(parse2 2)

>print $ rootParse 1
Struct3_var1 (Struct1 1 2 (StructParam 3 4) 5 6 7 8 (StructParam 9 0))

>print $ rootParse 2
Prelude.undefined

-- but I want something like this
>print $ rootParse 2
Struct3_var2("undefined")

Answer 1

You could add “Unimplemented” as a constructor in your data type, have a default case for all parses that ends in a definition of Unimplemented, and then the show instance will work trivially.

data Exp = Literal Int
         | Variable String
         | Operator Op Exp Exp
         | Unimplemented

    deriving Show