Towards Idris Version 1.0
idris-lang.org
idris-lang.org
The state machine stuff seems to mostly be concerned with using types to keep track of state, in terms of which actions are available or unavailable, e.g. you can't close a file unless it's open, you can't read from a file unless it's open, you can't run an expression unless it closes all of its files, etc.
There's certainly overlap, but I'm not sure whether one subsumes the other.
Intuitively, I like to think of it as:
- Extensible effects describe what we permit (read from console, print to console) - Indexed monads describe what we demand (you must auth before you can access db, you must open file before you can read it)
I'm actually working on doing a version of this in Haskell. You can see Oleg's version of it here: http://okmij.org/ftp/Haskell/extensible/ParamEff1.hs
I may have a more informed opinion after I read the states-all-the-way paper.
Normal extensible effects looks like:
data Union :: [Type] -> Type -> Type where
...
data Free :: (Type -> Type) -> Type -> Type where
...
type Eff fs = Free (Union fs)
class Inject f fs where
inj :: f a -> Union fs a
data File :: Type -> Type where
Open :: FilePath -> File ()
Close :: FilePath -> File ()
send :: (Inject f fs) => f a -> Eff fs a
-- The inferred type would be more general
openClose :: Eff '[File] ()
openClose = do
send (Open "foo.txt")
send (Close "foo.txt")
data HList :: [Type] -> Type where
Nil :: HList '[]
(:>) :: a -> HList as -> HList (a ': as)
data Union :: HList fs -> HList is -> HList is -> Type -> Type where
...
data Free :: (is -> is -> Type -> Type) -> is -> is -> Type -> Type where
...
type Eff fs = Free (Union fs)
class Inject f i o fs is os where
inj :: f i o a -> Union fs is os a
data FileStatus = Open | Closed
data File :: FileStatus -> FileStatus -> Type -> Type where
Open :: FilePath -> File 'Closed 'Open ()
Close :: FilePath -> File 'Open 'Closed ()
send :: (Inject f i o fs is os) => f i o a -> Eff fs is os a
-- The inferred type would be more general
openClose :: Eff (File ':> 'Nil) ('Closed ':> 'Nil) ('Closed ':> Nil) ()
openClose = do
send (Open "foo.txt")
send (Close "foo.txt")
The proliferation of type params looks intimidating from a user perspective though - is there some way to make it easier on folks? Would it look nicer in a language with row polymorphism perhaps?
There are two main differences at the moment:
- Everything has to be labelled - There's no 'implicit' lifting of smaller sets of states
The first is annoying for things like Console IO, so I might change that. The second means that we have a much better chance of providing decent error messages, one day.
I played with Idris quite a bit in the early days. I started spending more time in Coq around the time Idris gained its proof search mechanisms (I'm still not sure how they work!), but Idris has always been a more 'realistic' language for development (as opposed to proof engineering).
I think I spoiled myself with dependent types, as all of my Haskell projects seem to hit a wall which dependent types can trivially solve (e.g. calculating a type or a TypeRep based on incoming JSON data)
I don't think it can manage "full" dependent types, for example situations I often find myself in where I want to calculate a type from incoming data, e.g. something like:
parseJSON : String -> Maybe JSON
parseJSON = ...
parseMyData : JSON -> Maybe MyData
parseMyData = ...
HasSomeProperty : MyData -> Type
HasSomeProperty x = case x of
MyCase1 -> ...
...
someProcessing : (x : MyData) -> HasSomeProperty x -> ResultOfProcessing x
someProcessing x p = ...
This sort of code is trivial in a dynamically typed language: there's no requirement to prove anything, so we don't need any "HasSomeProperty x" values at all: we just go ahead and use whatever code we like, and hope that we didn't mess something up.
We hit problems when we want to write our program in a language like Haskell with a (relatively) weak type system: the type system quite rightly forbids us from writing the unsafe version that dynamic languages would accept, but the language isn't expressive enough for us to (easily) encode the proofs required to convince the type checker that what we're doing is safe.
I don't understand how you could possibly write any non-trivial (e.g. just unit-returning) code in the body of "someProcessing" with having some sort of proof object (generated by HasSomeProperty) with which to extract whatever you'd be interested in. As the example is written you'd only have a Type, but I don't see how that's enough.
Can you explain or perhaps give an example of what a "someProcessing" function body might look like?
As somebody with familiarity with Haskell in the small but not in large applications, is it not similarly trivial to write in Haskell with the same level of safety that dynamic languages give you (i.e. explosion at runtime)?
Consider as an example the following safe lookup function (standard Haskell)
lookup :: Eq a => a -> [(a, b)] -> Maybe b
lookup _ [] = Nothing
lookup x ((a, b):abs)
| a == x = Just b
| otherwise = lookup x abs
lookup :: Eq a => a -> [(a, b)] -> b
lookup x ((a, b):abs)
| a == x = b
| otherwise = lookup x abs
Presumably you mean "better", not "worse". Well, it's better because you will (or should) get a compiler warning (or ideally an error) about the latter version.
I didn't mean to imply that dynamic languages are "better" because they allow unchecked combinations of everything with everything else. Although I also didn't mean to imply that they're strictly "worse", precisely because there are things we can express easily in dynamic languages which take effort to reproduce in something like Haskell.
My main reason to mention dynamic languages was as motivation for why you might want to write such code in the first place: using dependent types to, say, prove Euclid's theorem of the infinitude of primes, would probably not motivate many software developers to give Idris a try. Examples like the type-safe printf mentioned in a sibling comment would presumably be much better motivators: we compute a type based on the format string, so providing a string like "Hello world %f %s !" will produce a type `Float -> String -> String` which accepts the required Float and String parameters, then returns the resulting String.
Off-topic, but related to undefined in Haskell, I love holes in Idris as a way to represent incomplete programs. I'd love if it somebody could take them one step further and have it so that if a hole is evaluated at runtime, a prompt came up and said "Well this is the input, what do you think the output should be?". You type it in, the program keeps going and a unit test is generated and put away somewhere for you to make sure that later when you implement it, you can help verify that you're doing it right.
This makes sense on its own, but seems a bit silly when we consider that completely removing the signature will fix the warning ;)
For some cases, this is pretty simple to implement with unsafePerformIO - I've done that once or twice. Not a sin at all in development.
One of my bigger interests in Haskell right now is the Frames library but I've honestly been struggling with using and understanding Template Haskell to generate Types.
I haven't had a ton of time so it isn't all TH's fault either. I've also read the Template Haskell paper and the entire system seems well thought out, I suppose there just aren't enough practical tutorials I can mess about with until I understand.
Thankfully for our purposes there's an escape hatch: the values we provide won't be evaluated, so they can be `undefined`, and the `TypeRep`s generated from these values are only used for determining whether two types are (intensionally) equal or different.
My solution was to define two new types, `Z` and `S a`, which can be used as peano numerals. We convert all of the distinct types in our JSON into distinct peano numeral types (`replaceTypes`), we generate `TypeRep` values for these numeral types (`getRep`), fabricate values of these types as arguments to the underlying library (`getVal`), then switch back all of the types which occur in the result (`restoreTypes`).
Very ugly, but it works, thanks to the wiggle room I had based on what these types and values would be used for.
Other projects have been even more icky, e.g. sending strings of auto-generated Haskell code through `runhaskell` [2] and throwing auto-generated template haskell into GHCi to see what sticks [3].
I don't recommend any of this for production code :)
[1] http://hackage.haskell.org/package/reduce-equations
[2] http://hackage.haskell.org/package/nix-eval
[3] http://chriswarbo.net/git/annotatedb/branches/master/annotat...
Does anyone know if Idris improves on this aspect of Haskell?
Idris on the other hand is still very much an experimental project more focused on practical applications of dependent type theory than things like performance. However, with enough work done on its compiler, its strictness and the abundance of type information might allow it to be eventually more performant. Though, the lack of type erasure might negatively impact performance as well depending on how types work at runtime...
However, GHC is a phenomenal compiler, and can outperform Idris for many things; this is natural, as it's seen a lot of interest in terms of producing performant code.
There are still some gotchas in Idris code. The biggest one that I can think of is indexing some type with data that doesn't manage to get erased (the usual type erasure algorithm is pretty aggressive, but it can't remove everything). Some of the most useful types for programming (as in, proving theorems about the code) are wonderfully inefficient (as an example, the inductive definition of natural number is an empty linked list - taking up plenty of space in pointers, and killing your cache whenever it's accessed - Idris knows about Nat, but not about other types). Operations on the data, which might look very efficient, can also end up operating on the indices, which might not be.
Here's the docs explaining the possible hiccup with erasure: http://docs.idris-lang.org/en/latest/reference/erasure.html
I am eager to point out, how lazy is to call lazy strict.
(Caveat: I've never written ATS, and although I've written in Coq and Idris, every time I look at ATS code it looks like complete gibberish).
To make things worse, I don't think ATS has any inference either, so all of this must be written explicitly. Idris, Agda, Coq, etc. can infer types and values, if they're unambiguous.
For example:
Definition Prime p := forall n m, n * m = p -> n = 1 \/ m = 1.
The seminal linearly typed language is Linear Lisp, which is almost incomprehensible, and also a proof-of-concept which doesn't do anything useful. Classic paper here:
http://home.pipeline.com/~hbaker1/LinearLisp.html
The closest thing to a modern language that uses them exclusively (Rust doesn't, by the way) is LinearML, which was an experimental toy, now abandoned. Tutorial here, which may be interesting:
https://github.com/pikatchu/LinearML/wiki/Tutorial
I asked about it on StackOverflow a while back, and it collected a bunch of useful resources, including links to some of the seminal papers:
http://stackoverflow.com/questions/5065861/programming-langu...
(Still looking for a modern pure linear typed language, by the way.)
Also, in my experience, most Rust programs tend to stash stuff in reference-counted boxes whenever calculating lifetime gets complex; which is reasonable enough (C++ does the same thing), but it is basically garbage collection, and I'd like to find an expressive, functional, non-GCd language which doesn't require this.
I do appreciate how you can use dependant types as much or as little as you like, so in some ways it can actually be considered a simpler and easier to learn Haskell due to having strict evaluation.
So I wouldn't count on too much momentum. ;)