CODEWITHSUNDEEP
stringhaskelltexttype-conversion
ruben.moor
ruben.moor

Reputation: 1965

Most elegant way to do string conversion in Haskell

See this related SO question: Automatic conversion between String and Data.Text in haskell

Given a string of type Text, I want to produce a lazy bytestring.

This works, but I wondered whether it's optimal, given the fact that both Text and the lazy bytestring have the property of being "string-like" and I still use the not-generic unpack:

import qualified   Data.ByteString.Lazy (ByteString)
import             Data.Text            (Text, unpack)
import             Data.String          (fromString)
import             Data.Text            (unpack)

convert :: IsString str => Text -> str
convert = fromString . unpack

I found the package string-conversions that offers the polymorphic function

 convertString :: a -> b

as part of the ConvertibleStrings typeclass.

While it works fine, I am suspicious: Why would I need an extra package for that? Couldn't there be already a typeclass like IsString that offers a toString method and in combination a universal convert function fromString . toString?

Upvotes: 1

Views: 786

Answers (1)

ruben.moor
ruben.moor

Reputation: 1965

[Ok, while I was editing my question, a possible answer dawned to me]

On the hackage-page of string-conversions it says:

Assumes UTF-8 encoding for both types of ByteStrings.

So there are assumptions that go along with conversions and a universal conversion of string-like types might not be desirable. Also performance probably depends on the input and output types and a universal conversion would pretend that it's all the same.

So my take on best practice is now this, being explicit rather than polymorphic:

import           Data.ByteString.Lazy (ByteString)
import qualified Data.ByteString.Lazy as ByteString
import qualified Data.Text.Encoding   as Text

convert :: Text -> ByteString
convert = ByteString.fromStrict . Text.encodeUtf8

Upvotes: 2

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