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This blog post is to share a very rough first stab at a new prelude I played around with earlier this month. I haven't used it in any significant way, and haven't spent more than a few hours on it total. I wrote it because I knew it was the only way to get the idea out of my head, and am sharing it in case anyone finds the idea intriguing or useful.
The project is available
on Github at snoyberg/safe-prelude,
uploaded the Haddocks for easier reading
(though, be warned, they aren't well organized at all). The rest of
this post is just a copy of the
README.md file for the project.
This is a thought experiment in a different point in the alternative
prelude design space. After my
blog post on readFile,
I realized I was unhappy with the polymorphic nature of
I've been itching to try something else. I have a lot of hope for the
but wanted to play with this in the short term.
No partial functions, period. If a function can fail, its return type must express that. (And for our purposes:
IOfunctions with runtime exceptions are not partial.)
Choose best in class libraries and promote them.
textfit that bill, as an example. Full listing below.
Regardless of the versions of underlying libraries, this package will always export a consistent API, so that CPP usage should be constrained to just inside this package.
Use generalization (via type classes) when they are well established. For example:
Controversial Avoid providing list-specific functions. This connects to the parent point. Most of the time, I'd argue that lists are not the correct choice, and instead a
Vectorshould be used. There is no standard for sequence-like typeclasses (though many exist), so we're not going to generalize. But we're also not going to use a less efficient representation.
I was torn on this, but decided in favor of leaving out functions initially, on the basis that it's easier to add something in later rather than remove it.
Encourage qualified imports with a consistent naming scheme. This is a strong departure from classy-prelude, which tried to make it unnecessary to use qualified imports. I'll save my feelings about qualified imports for another time, this is just a pragmatic choice given the other constraints.
Export any non-conflicting and not-discouraged names from this module that make sense, e.g.
This list may fall out of date, so check the
.cabal file for a
current and complete listing. I'm keeping this here to include
reasoning for some libraries:
text, despite some complaints, are clearly the most popular representation for binary and textual data, respectively
unordered-containersare both commonly used. Due to lack of generalization, this library doesn't expose any functions for working with their types, but they are common enough that adding the dependency just for exposing the type name is worth it
safe-exceptionshides the complexity of asynchronous exceptions, and should be used in place of
mtlare clear winners in the monad transformer space, at least for now
- While young,
sayhas been very useful for me in avoiding interleaved output issues
- Others without real competitors:
Packages I considered but have not included yet:
stmis an obvious winner, and while I use it constantly, I'm not convinced everyone else uses it as much as I do. Also, there are some questions around generalizing its functions (e.g.,
atomicallycould be in
MonadIO), and I don't want to make that decision yet.
stm-chansfalls into this category too
asyncis an amazing library, and in particular the
Concurrentlybits are an easy win. I've left it out for now due to questions of generalizing to
Similar argument applies to
I didn't bother with exposing the
Vectortype... because which one would I expose? The
Vector? Unboxed? I could do the classy-prelude thing and define
type UVector = Data.Vector.Unboxed.Vector, but I'd rather not do such renamings.
Here are the recommend qualified imports when working with safe-prelude.
import qualified "bytestring" Data.ByteString as B import qualified "bytestring" Data.ByteString.Lazy as BL import qualified "text" Data.Text as T import qualified "text" Data.Text.Lazy as TL import qualified "containers" Data.Map.Strict as Map import qualified "containers" Data.Set as Set import qualified "unordered-containers" Data.HashMap.Strict as HashMap import qualified "unordered-containers" Data.HashSet as HashSet