Haskell – Data.Text vs String

haskellstringtext;

While the general opinion of the Haskell community seems to be that it's always better to use Text instead of String, the fact that still the APIs of most of maintained libraries are String-oriented confuses the hell out of me. On the other hand, there are notable projects, which consider String as a mistake altogether and provide a Prelude with all instances of String-oriented functions replaced with their Text-counterparts.

So are there any reasons for people to keep writing String-oriented APIs except backwards- and standard Prelude-compatibility and the "switch-making intertia"?
Are there possibly any other drawbacks to Text as compared to String?

Particularly, I'm interested in this because I'm designing a library and trying to decide which type to use to express error messages.

Best Answer

My unqualified guess is that most library writers don't want to add more dependencies than necessary. Since strings are part of literally every Haskell distribution (it's part of the language standard!), it is a lot easier to get adopted if you use strings and don't require your users to sort out Text distributions from hackage.

It's one of those "design mistakes" that you just have to live with unless you can convince most of the community to switch over night. Just look at how long it has taken to get Applicative to be a superclass of Monad – a relatively minor but much wanted change – and imagine how long it would take to replace all the String things with Text.

To answer your more specific question: I would go with String unless you get noticeable performance benefits by using Text. Error messages are usually rather small one-off things so it shouldn't be a big problem to use String.

On the other hand, if you are the kind of ideological purist that eschews pragmatism for idealism, go with Text.

* I put design mistakes in scare quotes because strings as a list-of-chars is a neat property that makes them easy to reason about and integrate with other existing list-operating functions.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C++ – How to iterate over the words of a string

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