# Is the Maybe Monad Instance ever good?

**URL:** <https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133>\
**Category:** Learn\
**Created:** [October 18, 2025, 4:59pm UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133 "2025-10-18T16:59:10Z")\
**Posts on this page:** 6\
**Page:** 2

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**Author:** ![jackdk](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@jackdk](https://discourse.haskell.org/u/jackdk)\
**Post date:** [October 25, 2025, 8:19pm UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/21 "2025-10-25T20:19:59Z")

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I wouldn’t call that an abuse. To my eyes, `MonadFail` is for pattern-match failures and doing general error reporting with it is the real abuse.

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**Author:** ![Swordlash](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/swordlash/32/5817_2.png) [@Swordlash](https://discourse.haskell.org/u/Swordlash)\
**Post date:** [October 26, 2025, 1:55pm UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/22 "2025-10-26T13:55:10Z")

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Well it looks like little cheating to write `X a ← pure xOrY` since I need to wrap my thing in `Just` just to immediately unwrap and pattern match on it.

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**Author:** ![jackdk](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@jackdk](https://discourse.haskell.org/u/jackdk)\
**Post date:** [October 26, 2025, 10:25pm UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/23 "2025-10-26T22:25:08Z")

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I was surprised to discover that `let` in a `do`-block doesn’t desugar into `MonadFail` calls, and that this function is partial:

```haskell
data XOrY = X Int | Y Bool deriving Show
maybeDoubleX :: XOrY -> Maybe Int
maybeDoubleX xOrY = do
  let X x = xOrY
  Just $ x * 2

```

The “trick” you describe also works well when writing list comprehensions, FWIW. This can sometimes be really handy when traversing lists containing values of a sum type.

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**Author:** ![treblacy](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/treblacy/32/1782_2.png) [@treblacy](https://discourse.haskell.org/u/treblacy)\
**Post date:** [October 27, 2025, 6:38pm UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/24 "2025-10-27T18:38:24Z")

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I was thinking: Have the best/worst of both worlds—go MonadComprehension! \>:D

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**Author:** ![enlico](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/enlico/32/5600_2.png) [@enlico](https://discourse.haskell.org/u/enlico)\
**Post date:** [November 14, 2025, 8:46am UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/25 "2025-11-14T08:46:22Z")

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If you have a text comparison tool that shows results like this:

 ![image](https://us1.discourse-cdn.com/flex002/uploads/haskell/original/2X/f/faa8c1158cfa1a77d279d4f6173b556ec7358178.png)

one way to model that result is to have it be a `[[Maybe String]]`, where both layers of `[]` could actually be something better in producation code, e.g. the outer could be a some vector type, and the inner one some fixed-length vector type (fixed in the case that your application only supports 2-way diff, for instance).

In this case, I think `Maybe` fits be bill: `Nothing` is the way you model those grey placeholder lines between 6 and 7 on the left and between 4 and 5 on the right. There’s no additional information to be tied to those placeholders, as far as text alignment goes.

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**Author:** ![enlico](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/enlico/32/5600_2.png) [@enlico](https://discourse.haskell.org/u/enlico)\
**Post date:** [November 19, 2025, 1:25am UTC](https://discourse.haskell.org/t/is-the-maybe-monad-instance-ever-good/13133/26 "2025-11-19T01:25:23Z")

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Well, actually @sellout just made me realize that this usecase I described is a usecase for `Maybe` as a type, not for its `Monad` instance. I suppose I haven’t answered the OP’s question 😅

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