4 ms·

Please don't use Either to carry errors, it's really silly to use a language with a strong type system and then rely on convention to represent the error on the

Please don't use Either to carry errors, it's really silly to use a language with a strong type system and then rely on convention to represent the error on the correct side of the monad. Is error left or right again? That's right, you shouldn't have to remember. Use a more specific type for this, some GADT with clearly named instances (e.g. "Error" and "Value").

nemetroid 9y ago
Right is the right result and Left is the wrong result. That's about as easy it gets when it comes to remembering things. There is already a strong convention for using Either for this purpose and it's built into the standard library, since the Monad instance is designed to allow you to do this. If you create your own data type for this, you are designing yourself into the Monad transformer hell you mention in another comment.
- hota_mazi 9y ago
  > That's about as easy it gets when it comes to remembering things I use a statically typed language so I have fewer things to remember because the compiler does the bookkeeping for me. `Either` is a generic variant record, it should not be used to carry errors. And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs.
  - taeric 9y ago
    To be fair, this is more relying on a pun than it is convention. I don't think this changes your point, though.
  - nemetroid 9y ago
    > `Either` is a generic variant record, it should not be used to carry errors. The name suggests that, but the Monad instance suggests that Either is an asymmetric data type, where the left and right branches carry different meaning. > And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs. If the Either branches were named `Error` and `Success`, using `Error` to carry errors would be just as much of a convention as using `Left` for that purpose (a more sensible convention, but nonetheless a convention). The only way you'd get around that is if you'd somehow encode in the language what is and what isn't an error and use that to statically force correct branch usage.
dllthomas 9y ago
I agree that would be a good convention to move away from - I don't think it takes a GADT, though; a regular data type should be fine. For my part I think we should also deprecate the Functor (and dependent) instances for Either and tuple - but I know there are some who strongly disagree and I don't care all that much.

nemetroid 9y ago

Right is the right result and Left is the wrong result. That's about as easy it gets when it comes to remembering things. There is already a strong convention for using Either for this purpose and it's built into the standard library, since the Monad instance is designed to allow you to do this. If you create your own data type for this, you are designing yourself into the Monad transformer hell you mention in another comment.

hota_mazi 9y ago
> That's about as easy it gets when it comes to remembering things I use a statically typed language so I have fewer things to remember because the compiler does the bookkeeping for me. `Either` is a generic variant record, it should not be used to carry errors. And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs.
- taeric 9y ago
  To be fair, this is more relying on a pun than it is convention. I don't think this changes your point, though.
- nemetroid 9y ago
  > `Either` is a generic variant record, it should not be used to carry errors. The name suggests that, but the Monad instance suggests that Either is an asymmetric data type, where the left and right branches carry different meaning. > And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs. If the Either branches were named `Error` and `Success`, using `Error` to carry errors would be just as much of a convention as using `Left` for that purpose (a more sensible convention, but nonetheless a convention). The only way you'd get around that is if you'd somehow encode in the language what is and what isn't an error and use that to statically force correct branch usage.

dllthomas 9y ago

I agree that would be a good convention to move away from - I don't think it takes a GADT, though; a regular data type should be fine. For my part I think we should also deprecate the Functor (and dependent) instances for Either and tuple - but I know there are some who strongly disagree and I don't care all that much.