Solved: guards inside guards

Guards in Haskell is a feature that adds a bit more flexibility when defining functions. They allow you to specify various conditions and run different pieces of code depending on whether these conditions are met. Parallels can be made with conditional statements one might see in other languages, but with a more elegant and concise implementation. Haskell is a pure functional language, which means side effects like changing data elsewhere in the program are prohibited. Guards provide a way to handle such conditions within this paradigm.

The Problem of Nested Guards

Often in Haskell, you’ll find the situation where a certain condition needs to be met, but then within that condition, another condition also needs to be met. However, these conditions aren’t always naturally expressed with ‘if’ and ‘else’ statements, or with a list of patterns. Here is where nested guards could be useful.

Initially, Haskell does not support nested guards in a way that would allow one guard to contain another directly. While pattern matching can nest indefinitely, guards are limited to a “flat” structure. Therefore, a different approach must be taken.

Solution to the Problem

To solve this issue and achieve the functionality of nested guards, engineers have devised the strategy of incorporating case expressions within guards. Case expressions in Haskell are essentially switches, evaluating an expression and passing control to the first branch that matches. They provide a certain level of nested logic when combined with guards.

checkNumber num =
| num > 0 = case (num `mod` 2 == 0) of
True -> “Positive and Even”
False -> “Positive and Odd”
| num < 0 = "Negative" | otherwise = "Zero" [/code] In the above code snippet, a function "checkNumber" is defined with one parameter "num". Within the guard, a case expression is used to further divide the condition. In cases where 'num' is a positive number, the function checks if it is even or odd.

Step-By-Step Explanation of the Code

  • The function “checkNumber” predicts the nature of a given number.
  • When the function is invoked, the parameter passed is evaluated by the guards.
  • The first guard checks if the number is greater than zero. If that is true, a case expression is invoked.
  • The case expression checks if the number is even or odd and returns the appropriate string.
  • If the number is not positive, the guard moves to the next condition, which checks if it is negative. If true, “Negative” is returned.
  • If the number is not positive or negative, it must be zero, and hence “Zero” is returned.

The strategy of using case expressions within guards provides a way to imitate the desired nested guard functionality. This concept is key for code clarity and managing the program flow in an intuitive manner within the constraints of functional programming.


Overall, in pure functional languages like Haskell, managing program flow and conditions can often be a challenge. Guards and case expressions can be used effectively to provide rich, intuitive condition logic and control flow, keeping the code clear, easy to understand and maintain.

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