Question

I have googled about trying to find a simple example of Haskell being used to define a simple language but to no avail.

I did find this post on stackoverflow giving a similar example, but when i implemented it, it didn’t appear to work:

Haskell – How to best to represent a programming language's grammar?

an example expression in this language would be:

if true then x else (if false then y else true) Your Haskell data type
would look something like this:

data Expr = Var String
          | Lit Bool
          | If Expr Expr Expr

however, when i entered if true then x else (if false then y else true) into the console as input, it complained about “x” not being interpretable. It also didn’t like true and false.

EDIT: I did derive “show” also, at the end

Answer 1

If you want to represent if true then x else (if false then y else true) in your little language, you need to use the following expression:

If (Lit True) (Var "x") (If (Lit False) (Var "y") (Lit True))

If, as you say, you derived Show this will also show exactly as entered.

I’m not sure what more you want to do! Possible further things to try are:

• Writing an evaluation function.
• Writing your own Show instance to get the printed representation to look nicer.
• Writing a parser for this little language.