I wrote the following logical expression evaluator. It works for simple 2-member expressions, and it runs but produces a fault for expression containing other expressions as the second/first member. Here's my code.

```
data Expression = Literal Bool | Operation Operator Expression Expression
data Operator = AND | OR
eval :: Expression -> Bool
eval (Literal x) = x
eval (Operation AND (Literal x) (Literal y))
| x == True && y == True = True
| otherwise = False
eval (Operation OR (Literal x) (Literal y))
| x == False && y == False = False
| otherwise = True
```

It works correctly when called with this input:

```
main = do
print $ eval (Operation OR (Literal False) (Literal False))
```

But produces an error when called with this input:

```
main = do
print $ eval( Operation OR (Literal True) (Operation AND (Literal True) (Literal False)) )
```

Answer:

You're making `eval`

a bit too low-level. By including `Literal`

s in the signature. A better way to do this is, is using recursion:

```
eval :: Expression -> Bool
eval (Literal x) = x
eval (Operation AND x y) = (eval x) && (eval y)
eval (Operation OR x y) = (eval x) || (eval y)
```

In other words call `eval`

on the *right-hand-side*. In case it is a `Literal`

, it will immediately resolve to the correct value, in case it is a cascaded expression, it will resolve that `Operation _ _ _`

as well.

In general it is not advisable to start cascaded pattern matching (ok, sometimes it is useful). In that case you should at least ask yourself if there is no more elegant solution.

This piece of code easily shows that the function is *total* (regardless of the input, it will always generate a result). That's not the case for your code. Always try to perform a *totality-check*.

In case the number of `Operation`

s will increase significantly, you better separate concerns into a `handler :: Operation -> Bool -> Bool -> Bool`

function and the `eval`

function. Something like:

```
data Expression = Literal Bool | Operation Operator Expression Expression
data Operator = AND | OR | XOR
handler :: Operation -> Bool -> Bool -> Bool
handler AND = (&&)
handler OR = (||)
handler XOR = xor
where xor True False = True
xor False True = True
xor _ _ = False
eval :: Expression -> Bool
eval (Literal x) = x
eval (Operation o x y) = (handler o) (eval x) (eval y)
```

In case you need to handle a `NOT`

, that's another type of expression:

```
data Expression = Literal Bool | Operation Operator Expression Expression | OperationU OperatorU Expression
```

`OperatorU`

is here an unary operator. For instance:

```
data OperatorU = ID | NOT
```

with `ID`

the identity. Now in that case you can define a second handler:

```
handlerU :: OperatorU -> Bool -> Bool
handlerU ID = id
handlerU NOT = not
```

and then `eval`

reads:

```
eval :: Expression -> Bool
eval (Literal x) = x
eval (Operation o x y) = (handler o) (eval x) (eval y)
eval (OperationU o x) = (handlerU o) (eval x)
```

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