Why not: class isString1 a where fromString1 :: NonEmpty Char -> a On 02/09/17 22:08, Artyom wrote:
tl;dr
We have |instance IsString [Char]| and |instance IsList (NonEmpty a)|. Let’s also have |IsString (NonEmpty Char)|.
Background
|IsString| is a class that is used with the |-XOverloadedStrings| extension to support string literals of types other than |String| - for instance, with the |IsString Text| instance in scope, you can write |"foobar" :: Text| and it will compile.
For reference, the standard libraries supply the following instances of |IsString|:
|instance (a ~ Char) => IsString [a] instance IsString a => IsString (Const a b) instance IsString a => IsString (Identity a) |
Proposal
I propose adding a new instance of |IsString| for |NonEmpty| lists of characters. |NonEmpty| has been in |base| starting from version 4.9, and for that reason people are starting to use it more often - e.g. the popular |megaparsec| library defines its custom error type like this:
|data ErrorItem t = Tokens (NonEmpty t) -- ^ Non-empty stream of tokens | Label (NonEmpty Char) -- ^ Label (cannot be empty) | EndOfInput -- ^ End of input |
Here |NonEmpty Char| stands for “non-empty string”. Without the |IsString| instance users are forced to write non-empty strings in an inconvenient and awkward-looking way (e.g. |Label ('f' :| "oobar")|); the instance makes |Label "foobar"| an acceptable notation, thus making the |NonEmpty Char| type more viable for use in libraries and user-facing APIs.
Implementation
Here’s a sample implementation:
|instance (a ~ Char) => IsString (NonEmpty a) where fromString (a:as) = a :| as fromString "" = errorWithoutStackTrace "NonEmpty.fromString: empty string" |
This mirrors the |IsList| instance for |NonEmpty|. (The reason I haven’t used |fromList| is that I want the error message to say “fromString” instead of “fromList”.)
If this is accepted, I can make a patch.
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