Why not:

class isString1 a where fromString1 :: NonEmpty Char -> a

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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