An IsString (NonEmpty Char) instance

Artyom yom at
Sat Sep 2 12:08:52 UTC 2017


We have |instance IsString [Char]| and |instance IsList (NonEmpty a)|.
Let’s also have |IsString (NonEmpty Char)|.


|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

|instance (a ~ Char) => IsString [a] instance IsString a => IsString
(Const a b) instance IsString a => IsString (Identity a) |


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.


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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