[lostmsu]

Imagine you're working on a compiler. You need to represent compile-time computed value of type Maybe Int (e.g. you are precomputing nullable integers).

You see 1 + null. So you have add: Maybe Int -> Maybe Int -> Maybe Int, that takes two precomputed values, and returns new precomputed value for the operation result.

However, you can't precompute Console.readInt().

For some expression, you can either be able to compute value at compile time, or not.

What is the output type of compileTimeCompute: Expr -> ???

[ndh2]

I don't understand your example. What does compile-time computed stuff have to do with readInt()?

I get that it might be possible to do that, use a Maybe Maybe T. But it's like an optional<bool> in C++. It can be done, it's just not a good idea. So if you design your system not to allow that in the first place, nothing of value was lost.

If you have specific error cases that you want to communicate, like "what was read from the console didn't parse as an int" as opposed to "the computation didn't find a result", then using the two values "Nothing" and "Just Nothing" as the two distinct values to encode that is not a sound design. Either you have meaning, or you have Nothing. Nothing shouldn't have any meaning attached to it.

[lostmsu]

> "what was read from the console didn't parse as an int"

I meant what you read from the console can not be computed at compile time.