.. _updates-index: ##################### Changes since Idris 1 ##################### Idris 2 is mostly backwards compatible with Idris 1, with some minor exceptions. This document describes the changes, approximately in order of likelihood of encountering them in practice. New features are described at the end, in Section :ref:`sect-new-features`. The Section :ref:`typedd-index` describes how these changes affect the code in the book `Type-Driven Development with Idris `_ by Edwin Brady, available from `Manning `_. .. note:: The documentation for Idris has been published under the Creative Commons CC0 License. As such to the extent possible under law, *The Idris Community* has waived all copyright and related or neighboring rights to Documentation for Idris. More information concerning the CC0 can be found online at: http://creativecommons.org/publicdomain/zero/1.0/ New Core Language: Quantities in Types ====================================== Idris 2 is based on `Quantitative Type Theory (QTT) `_, a core language developed by Bob Atkey and Conor McBride. In practice, this means that every variable in Idris 2 has a *quantity* associated with it. A quantity is one of: * ``0``, meaning that the variable is *erased* at run time * ``1``, meaning that the variable is used *exactly once* at run time * *Unrestricted*, which is the same behaviour as Idris 1 For more details on this, see Section :ref:`sect-multiplicities`. In practice, this might cause some Idris 1 programs not to type check in Idris 2 due to attempting to use an argument which is erased at run time. Erasure ------- In Idris, names which begin with a lower case later are automatically bound as implicit arguments in types, for example in the following skeleton definition, ``n``, ``a`` and ``m`` are implicitly bound: .. code-block:: idris append : Vect n a -> Vect m a -> Vect (n + m) a append xs ys = ?append_rhs One of the difficulties in compiling a dependently typed programming language is deciding which arguments are used at run time and which can safely be erased. More importantly, it's one of the difficulties when programming: how can a programmer *know* when an argument will be erased? In Idris 2, a variable's quantity tells us whether it will be available at run time or not. We can see the quantities of the variables in scope in ``append_rhs`` by inspecting the hole at the REPL: :: Main> :t append_rhs 0 m : Nat 0 a : Type 0 n : Nat ys : Vect m a xs : Vect n a ------------------------------------- append_rhs : Vect (plus n m) a The ``0`` next to ``m``, ``a`` and ``n`` mean that they are in scope, but there will be ``0`` occurrences at run-time. That is, it is *guaranteed* that they will be erased at run-time. This does lead to some potential difficulties when converting Idris 1 programs, if you are using implicit arguments at run time. For example, in Idris 1 you can get the length of a vector as follows: .. code-block:: idris vlen : Vect n a -> Nat vlen {n} xs = n This might seem like a good idea, since it runs in constant time and takes advantage of the type level information, but the trade off is that ``n`` has to be available at run time, so at run time we always need the length of the vector to be available if we ever call ``vlen``. Idris 1 can infer whether the length is needed, but there's no easy way for a programmer to be sure. In Idris 2, we need to state explicitly that ``n`` is needed at run time .. code-block:: idris vlen : {n : Nat} -> Vect n a -> Nat vlen xs = n (Incidentally, also note that in Idris 2, names bound in types are also available in the definition without explicitly rebinding them.) This also means that when you call ``vlen``, you need the length available. For example, this will give an error .. code-block:: idris sumLengths : Vect m a -> Vect n a —> Nat sumLengths xs ys = vlen xs + vlen ys Idris 2 reports:: vlen.idr:7:20--7:28:While processing right hand side of Main.sumLengths at vlen.idr:7:1--10:1: m is not accessible in this context This means that it needs to use ``m`` as an argument to pass to ``vlen xs``, where it needs to be available at run time, but ``m`` is not available in ``sumLengths`` because it has multiplicity ``0``. We can see this more clearly by replacing the right hand side of ``sumLengths`` with a hole... .. code-block:: idris sumLengths : Vect m a -> Vect n a -> Nat sumLengths xs ys = ?sumLengths_rhs ...then checking the hole's type at the REPL:: Main> :t sumLengths_rhs 0 n : Nat 0 a : Type 0 m : Nat ys : Vect n a xs : Vect m a ------------------------------------- sumLengths_rhs : Nat Instead, we need to give bindings for ``m`` and ``n`` with unrestricted multiplicity .. code-block:: idris sumLengths : {m, n : _} -> Vect m a -> Vect n a —> Nat sumLengths xs ys = vLen xs + vLen xs Remember that giving no multiplicity on a binder, as with ``m`` and ``n`` here, means that the variable has unrestricted usage. If you're converting Idris 1 programs to work with Idris 2, this is probably the biggest thing you need to think about. It is important to note, though, that if you have bound implicits, such as... .. code-block:: idris excitingFn : {t : _} -> Coffee t -> Moonbase t ...then it's a good idea to make sure ``t`` really is needed, or performance might suffer due to the run time building the instance of ``t`` unnecessarily! One final note on erasure: it is an error to try to pattern match on an argument with multiplicity ``0``, unless its value is inferrable from elsewhere. So, the following definition is rejected .. code-block:: idris badNot : (0 x : Bool) -> Bool badNot False = True badNot True = False This is rejected with the error:: badnot.idr:2:1--3:1:Attempt to match on erased argument False in Main.badNot The following, however, is fine, because in ``sNot``, even though we appear to match on the erased argument ``x``, its value is uniquely inferrable from the type of the second argument .. code-block:: idris data SBool : Bool -> Type where SFalse : SBool False STrue : SBool True sNot : (0 x : Bool) -> SBool x -> Bool sNot False SFalse = True sNot True STrue = False Experience with Idris 2 so far suggests that, most of the time, as long as you're using unbound implicits in your Idris 1 programs, they will work without much modification in Idris 2. The Idris 2 type checker will point out where you require an unbound implicit argument at run time - sometimes this is both surprising and enlightening! Linearity --------- Full details on linear arguments with multiplicity ``1`` are given in Section :ref:`sect-multiplicities`. In brief, the intution behind multiplicity ``1`` is that if we have a function with a type of the following form... .. code-block:: idris f : (1 x : a) -> b ...then the guarantee given by the type system is that *if* ``f x`` *is used exactly once, then* ``x`` *is used exactly once* in the process. Prelude and ``base`` libraries ============================== The Prelude in Idris 1 contained a lot of definitions, many of them rarely necessary. The philosophy in Idris 2 is different. The (rather vaguely specified) rule of thumb is that it should contain the basic functions required by almost any non-trivial program. This is a vague specification since different programmers will consider different things absolutely necessary, but the result is that it contains: - Anything the elaborator can desugar to (e.g. tuples, ``()``, ``=``) - Basic types ``Bool``, ``Nat``, ``List``, ``Stream``, ``Dec``, ``Maybe``, ``Either`` - The most important utility functions: ``id``, ``the``, composition, etc - Interfaces for arithmetic and implementations for the primitives and basic types - Basic ``Char`` and ``String`` manipulation - ``Show``, ``Eq``, ``Ord``, and implementations for all types in the prelude - Interfaces and functions for basic proof (``cong``, ``Uninhabited``, etc) - ``Semigroup``, ``Monoid`` - ``Functor``, ``Applicative``, ``Monad`` and related functions - ``Foldable``, ``Alternative`` and ``Traversable`` - ``Enum``, for list range syntax - Console ``IO`` Anything which doesn't fit in here has been moved to the ``base`` libraries. Among other places, you can find some of the functions which used to be in the prelude in: - ``Data.List`` and ``Data.Nat`` - ``Data.Maybe`` and ``Data.Either`` - ``System.File`` and ``System.Directory``, (file management was previously part of the prelude) - ``Decidable.Equality`` Smaller Changes =============== Ambiguous Name Resolution ------------------------- Idris 1 worked very hard to resolve ambiguous names, by type, even if this involved some complicated interaction with interface resolution. This could sometimes be the cause of long type checking times. Idris 2 simplifies this, at the cost of requiring more programmer annotations on ambiguous names. As a general rule, Idris 2 will be able to disambiguate between names which have different concrete return types (such as data constructors), or which have different concrete argument types (such as record projections). It may struggle to resolve ambiguities if one name requires an interface to be resolved. It will postpone resolution if a name can't be resolved immediately, but unlike Idris 1, it won't attempt significant backtracking If you get an ambiguous name error, generally the best approach is to give the namespace explicitly. You can also rebind names locally: .. code-block:: idris Main> let (::) = Prelude.(::) in [1,2,3] [1, 2, 3] One difficulty which remains is resolving ambiguous names where one possibility is an interface method, and another is a concrete top level function. For example, we may have: .. code-block:: idris Prelude.(>>=) : Monad m => m a -> (a -> m b) -> m b LinearIO.(>>=) : (1 act : IO a) -> (1 k : a -> IO b) -> IO b As a pragmatic choice, if type checking in a context where the more concrete name is valid (``LinearIO.(>>=)`` here, so if type checking an expression known to have type ``IO t`` for some ``t``), the more concrete name will be chosen. This is somehow unsatisfying, so we may revisit this in future! Modules, namespaces and export ------------------------------ The visibility rules, as controlled by the ``private``, ``export`` and ``public export`` modifiers, now refer to the visibility of names from other *namespaces*, rather than other *files*. So if you have the following, all in the same file... .. code-block:: idris namespace A private aHidden : Int -> Int aHidden x = x * 2 export aVisible : Int -> Int aVisibile x = aHidden x mamespace B export bVisible : Int -> Int bVisible x = aVisible (x * 2) ...then ``bVisible`` can access ``aVisible``, but not ``aHidden``. Records are, as before, defined in their own namespace, but fields are always visible from the parent namespace. Also, module names must now match the filename in which they're defined, with the exception of the module ``Main``, which can be defined in a file with any name. ``%language`` pragmas --------------------- There are several ``%language`` pragmas in Idris 1, which define various experimental extensions. None of these are available in Idris 2, although extensions may be defined in the future. ``let`` bindings ---------------- ``let`` bindings, i.e. expressions of the form ``let x = val in e`` have slightly different behaviour. Previously, you could rely on the computational behaviour of ``x`` in the scope of ``e``, so type checking could take into account that ``x`` reduces to ``val``. Unfortunately, this leads to complications with ``case`` and ``with`` clauses: if we want to preserve the computational behaviour, we would need to make significant changes to the way ``case`` and ``with`` are elaborated. So, for simplicity and consistency (and, realistically, because I don't have enough time to resolve the problem for ``case`` and ``with``) the above expression ``let x = val in e`` is equivalent to ``(\x => e) val``. So, ``let`` now effectively generalises a complex subexpression. If you do need the computational behaviour of a definition, it is now possible using local function definitions instead - see Section :ref:`sect-local-defs` below. ``auto``-implicits and Interfaces --------------------------------- Interfaces and ``auto``-implicit arguments are similar, in that they invoke an expression search mechanism to find the value of an argument. In Idris 1, they were implemented separately, but in Idris 2, they use the same mechanism. Consider the following *total* definition of ``fromMaybe``: .. code-block:: idris data IsJust : Maybe a -> Type where ItIsJust : IsJust (Just val) fromMaybe : (x : Maybe a) -> {auto p : IsJust x} -> a fromMaybe (Just x) {p = ItIsJust} = x Since interface resolution and ``auto``-implicits are now the same thing, the type of ``fromMaybe`` can be written as: .. code-block:: idris fromMaybe : (x : Maybe a) -> IsJust x => a So now, the constraint arrow ``=>`` means that the argument will be found by ``auto``-implicit search. When defining a ``data`` type, there are ways to control how ``auto``-implicit search will proceed, by giving options to the data type. For example: .. code-block:: idris data Elem : (x : a) -> (xs : List a) -> Type where [search x] Here : Elem x (x :: xs) There : Elem x xs -> Elem x (y :: xs) The ``search x`` option means that ``auto``-implicit search for a value of type ``Elem t ts`` will start as soon as the type checker has resolved the value ``t``, even if ``ts`` is still unknown. By default, ``auto``-implicit search uses the constructors of a data type as search hints. The ``noHints`` option on a data type turns this behaviour off. Totality and Coverage --------------------- ``%default covering`` is now the default status [Actually still TODO, but planned soon!] Build artefacts --------------- This is not really a language change, but a change in the way Idris saves checked files, and still useful to know. All checked modules are now saved in a directory ``build/ttc``, in the root of the source tree, with the directory structure following the directory structure of the source. Executables are placed in ``build/exec``. .. _sect-new-features: New features ============ As well as the change to the core language to use quantitative type theory, described above, there are several other new features. .. _sect-local-defs: Local function definitions -------------------------- Functions can now be defined locally, using a ``let`` block. For example, ``greet`` in the following example, which is defined in the scope of a local variable ``x``: .. code-block:: idris chat : IO () chat = do putStr "Name: " x <- getLine let greet : String -> String greet msg = msg ++ " " ++ x putStrLn (greet "Hello") putStrLn (greet "Bye") These ``let`` blocks can be used anywhere (in the middle of ``do`` blocks as above, but also in any function, or in type declarations). ``where`` blocks are now elaborated by translation into a local ``let``. However, Idris no longer attempts to infer types for functions defined in ``where`` blocks, because this was fragile. This may be reinstated, if we can come up with a good, predictable approach. Scope of implicit arguments --------------------------- Implicit arguments in a type are now in scope in the body of a definition. We've already seen this above, where ``n`` is in scope automatically in the body of ``vlen``: .. code-block:: idris vlen : {n : Nat} -> Vect n a -> Nat vlen xs = n This is important to remember when using ``where`` blocks, or local definitions, since the names in scope will also be in scope when declaring the *type* of the local definition. For example, the following definition, where we attempt to define our own version of ``Show`` for ``Vect``, will fail to type check: .. code-block:: idris showVect : Show a => Vect n a -> String showVect xs = "[" ++ showBody xs ++ "]" where showBody : Vect n a -> String showBody [] = "" showBody [x] = show x showBody (x :: xs) = show x ++ ", " ++ showBody xs This fails because ``n`` is in scope already, from the type of ``showVect``, in the type declaration for ``showBody``, and so the first clause ``showBody []`` will fail to type check because ``[]`` has length ``Z``, not ``n``. We can fix this by locally binding ``n``: .. code-block:: idris showVect : Show a => Vect n a -> String showVect xs = "[" ++ showBody xs ++ "]" where showBody : forall n . Vect n a -> String ... Or, alternatively, using a new name: .. code-block:: idris showVect : Show a => Vect n a -> String showVect xs = "[" ++ showBody xs ++ "]" where showBody : Vect n' a -> String ... Idris 1 took a different approach here: names which were parameters to data types were in scope, other names were not. The Idris 2 approach is, we hope, more consistent and easier to understand. Better inference ---------------- In Idris 1, holes (that is, unification variables arising from implicit arguments) were local to an expression, and if they were not resolved while checking the expression, they would not be resolved at all. In Idris 2, they are global, so inference works better. For example, we can now say: .. code-block:: idris test : Vect ? Int test = [1,2,3,4] Main> :t test Main.test : Vect (S (S (S (S Z)))) Int The ``?``, incidentally, differs from ``_`` in that ``_`` will be bound as an implicit argument if unresolved after checking the type of ``test``, but ``?`` will be left as a hole to be resolved later. Otherwise, they can be used interchangeably. Dependent case -------------- ``case`` blocks were available in Idris 1, but with some restrictions. Having better inference means that ``case`` blocks work more effectively in Idris 2, and dependent case analysis is supported. .. code-block:: idris append : Vect n a -> Vect m a -> Vect (n + m) a append xs ys = case xs of [] => ys (x :: xs) => x :: append xs ys The implicit arguments and original values are still available in the body of the ``case``. Somewhat contrived, but the following is valid: .. code-block:: idris info : {n : _} -> Vect n a -> (Vect n a, Nat) info xs = case xs of [] => (xs, n) (y :: ys) => (xs, n) Record updates -------------- Dependent record updates work, provided that all relevant fields are updated at the same time. Dependent record update is implemented via dependent ``case`` blocks rather than by generating a specific update function for each field as in Idris 1, so you will no longer get mystifying errors when trying to update dependent records! For example, we can wrap a vector in a record, with an explicit length field: .. code-block:: idris record WrapVect a where constructor MkVect purpose : String length : Nat content : Vect length a Then, we can safely update the ``content``, provided we update the ``length`` correspondingly: .. code-block:: idris addEntry : String -> WrapVect String -> WrapVect String addEntry val = record { length $= S, content $= (val :: ) } Generate definition ------------------- A new feature of the IDE protocol supports generating complete definitions from a type signature. You can try this at the REPL, for example, given our favourite introductory example... .. code-block:: idris append : Vect n a -> Vect m a -> Vect (n + m) a ...assuming this is defined on line 3, you can use the ``:gd`` command as follows: .. code-block:: idris Main> :gd 3 append append [] ys = ys append (x :: xs) ys = x :: append xs ys This works by a fairly simple brute force search, which tries searching for a valid right hand side, and case splitting on the left if that fails, but is remarkably effective in a lot of situations. Some other examples which work: .. code-block:: idris my_cong : forall f . (x : a) -> (y : a) -> x = y -> f x = f y my_curry : ((a, b) -> c) -> a -> b -> c my_uncurry : (a -> b -> c) -> (a, b) -> c append : Vect n a -> Vect m a -> Vect (n + m) a lappend : (1 xs : List a) -> (1 ys : List a) -> List a zipWith : (a -> b -> c) -> Vect n a -> Vect n b -> Vect n c This is available in the IDE protocol via the ``generate-def`` command. Chez Scheme target ------------------ The default code generator is, for the moment, `Chez Scheme `_. A Racket code generator is also available. There is not yet a way to plug in code generators as in Idris 1, but this is coming. To change the code generator, you can use the ``:set cg`` command: :: Main> :set cg racket Early experience shows that both are much faster than the Idris 1 C code generator, in both compile time and execution time (but we haven't done any formal study on this yet, so it's just anecdotal evidence).