FazBrowse GitHub Viewer | Trending |
URL:
| Home
Tools: [Download Repo ZIP]   [Original HTTPS Page]

Describe ambiguous matches in instance chains by rhendric · Pull Request #392 · purescript/documentation · GitHub

/ documentation Public

Describe ambiguous matches in instance chains #392

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
JordanMartinez merged 1 commit into from
Apr 27, 2022
Conversation 3 Commits 1 Checks 0 Files changed
Merged

Describe ambiguous matches in instance chains #392

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension .md  (1) All 1 file type selected
Viewed files
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Unified
Split
Hide whitespace
Diff view
Unified
Split
Hide whitespace
20 changes: 20 additions & 0 deletions language/Type-Classes.md
Show comments
View file
Open in desktop
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Show hidden characters
Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,26 @@ main = do
log $ myShow MysteryItem -- Invalid
```

When type variables are present in the constraint being solved, the question of whether an instance in a chain matches is a little subtle. A type variable in the constraint is treated existentially—it represents some concrete type, but the constraint solver isn't allowed to assume that it is any particular type. This means that, when evaluating an instance as a solution for a particular constraint, there are three possible outcomes for the instance: it can match, it can fail to match, or it can be ambiguous. An ambiguous instance is one that could match the constraint only if one or more of the variables in the constraint happened to represent a particular type.

milesfrain Jun 30, 2021

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Should spaces surround the hyphen/dash in existentially — it?

rhendric Jun 30, 2021

Copy link
Copy Markdown
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

The fixed-width font obscures it, but that's an em dash—like this. I see em dashes set off with spaces in newspapers, but for other writing I don't think spaces are typical.

Is a spaced hyphen generally used in place of an em dash in these docs? I can change it if so; the dash was just a reflexive choice.

```purescript
class MyShow a where
myShow :: a -> String

instance showStringTuple :: MyShow a => MyShow (Tuple String a) where
myShow (Tuple s a) = s <> ": " <> myShow a

else instance showA :: MyShow a where
myShow _ = "Invalid"

f :: forall l r. Tuple l r -> String
f = myShow -- error: no instance found
```

In this example, `f` needs an instance of `MyShow (Tuple l r)`. The `showStringTuple` is an ambiguous match for this constraint, because it would only match if `l` represented the type `String`. But `f` needs an implementation that will work for all `l`, so this can't be used as a full match.

Ambiguous instances are mostly treated the same as an instance that fails to match, with this exception. If an instance in an instance chain fails to match, the compiler will try the next instance in the chain. But if an instance in an instance chain is an ambiguous match, the compiler will not consider any more instances in that chain. In the above example, the compiler will report that no instance was found for `MyShow (Tuple l r)` in `f`, even though the `showA` instance could have been a match, because the ambiguous `showStringTuple` prevents `showA` from being considered.

## Multi-Parameter Type Classes

TODO. For now, see the section in [PureScript by Example](https://book.purescript.org/chapter6.html#multi-parameter-type-classes).
Expand Down
Back | FazBrowse Home | New Git URL