Frequently Asked Questions

What are the differences between Agda and Idris?

Like Idris, Agda is a functional language with dependent types, supporting dependent pattern matching. Both can be used for writing programs and proofs. However, Idris has been designed from the start to emphasise general purpose programming rather than theorem proving. As such, it supports interoperability with systems libraries and C programs, and language constructs for domain specific language implementation. It also includes higher level programming constructs such as interfaces (similar to type classes) and do notation.

Idris supports multiple back ends (C and JavaScript by default, with the ability to add more via plugins) and has a reference run time system, written in C, with a garbage collector and built-in message passing concurrency.

Is Idris production ready?

Idris is primarily a research tool for exploring the possibilities of software development with dependent types, meaning that the primary goal is not (yet) to make a system which could be used in production. As such, there are a few rough corners, and lots of missing libraries. Nobody is working on Idris full time, and we don’t have the resources at the moment to polish the system on our own. Therefore, we don’t recommend building your business around it!

Having said that, contributions which help towards making Idris suitable for use in production would be very welcome - this includes (but is not limited to) extra library support, polishing the run-time system (and ensuring it is robust), providing and maintaining a JVM back end, etc.

Why does Idris use eager evaluation rather than lazy?

Idris uses eager evaluation for more predictable performance, in particular because one of the longer term goals is to be able to write efficient and verified low level code such as device drivers and network infrastructure. Furthermore, the Idris type system allows us to state precisely the type of each value, and therefore the run-time form of each value. In a lazy language, consider a value of type Int:

thing : Int

What is the representation of thing at run-time? Is it a bit pattern representing an integer, or is it a pointer to some code which will compute an integer? In Idris, we have decided that we would like to make this distinction precise, in the type:

thing_val : Int
thing_comp : Lazy Int

Here, it is clear from the type that thing_val is guaranteed to be a concrete Int, whereas thing_comp is a computation which will produce an Int.

How can I make lazy control structures?

You can make control structures using the special Lazy type. For example, if...then...else... in Idris expands to an application of a function named ifThenElse. The default implementation for Booleans is defined as follows in the library:

ifThenElse : Bool -> (t : Lazy a) -> (e : Lazy a) -> a
ifThenElse True  t e = t
ifThenElse False t e = e

The type Lazy a for t and e indicates that those arguments will only be evaluated if they are used, that is, they are evaluated lazily.

Evaluation at the REPL doesn’t behave as I expect. What’s going on?

Being a fully dependently typed language, Idris has two phases where it evaluates things, compile-time and run-time. At compile-time it will only evaluate things which it knows to be total (i.e. terminating and covering all possible inputs) in order to keep type checking decidable. The compile-time evaluator is part of the Idris kernel, and is implemented in Haskell using a HOAS (higher order abstract syntax) style representation of values. Since everything is known to have a normal form here, the evaluation strategy doesn’t actually matter because either way it will get the same answer, and in practice it will do whatever the Haskell run-time system chooses to do.

The REPL, for convenience, uses the compile-time notion of evaluation. As well as being easier to implement (because we have the evaluator available) this can be very useful to show how terms evaluate in the type checker. So you can see the difference between:

Idris> \n, m => (S n) + m
\n => \m => S (plus n m) : Nat -> Nat -> Nat

Idris> \n, m => n + (S m)
\n => \m => plus n (S m) : Nat -> Nat -> Nat

Why can’t I use a function with no arguments in a type?

If you use a name in a type which begins with a lower case letter, and which is not applied to any arguments, then Idris will treat it as an implicitly bound argument. For example:

append : Vect n ty -> Vect m ty -> Vect (n + m) ty

Here, n, m, and ty are implicitly bound. This rule applies even if there are functions defined elsewhere with any of these names. For example, you may also have:

ty : Type
ty = String

Even in this case, ty is still considered implicitly bound in the definition of append, rather than making the type of append equivalent to...

append : Vect n String -> Vect m String -> Vect (n + m) String

...which is probably not what was intended! The reason for this rule is so that it is clear just from looking at the type of append, and no other context, what the implicitly bound names are.

If you want to use an unapplied name in a type, you have two options. You can either explicitly qualify it, for example, if ty is defined in the namespace Main you can do the following:

append : Vect n Main.ty -> Vect m Main.ty -> Vect (n + m) Main.ty

Alternatively, you can use a name which does not begin with a lower case letter, which will never be implicitly bound:

Ty : Type
Ty = String

append : Vect n Ty -> Vect m Ty -> Vect (n + m) Ty

As a convention, if a name is intended to be used as a type synonym, it is best for it to begin with a capital letter to avoid this restriction.

I have an obviously terminating program, but Idris says it possibly isn’t total. Why is that?

Idris can’t decide in general whether a program is terminating due to the undecidability of the Halting Problem. It is possible, however, to identify some programs which are definitely terminating. Idris does this using “size change termination” which looks for recursive paths from a function back to itself. On such a path, there must be at least one argument which converges to a base case.

  • Mutually recursive functions are supported
  • However, all functions on the path must be fully applied. In particular, higher order applications are not supported
  • Idris identifies arguments which converge to a base case by looking for recursive calls to syntactically smaller arguments of inputs. e.g. k is syntactically smaller than S (S k) because k is a subterm of S (S k), but (k, k) is not syntactically smaller than (S k, S k).

If you have a function which you believe to be terminating, but Idris does not, you can either restructure the program, or use the assert_total function.

When will Idris be self-hosting?

It’s not a priority, though not a bad idea in the long run. It would be a worthwhile effort in the short term to implement libraries to support self-hosting, such as a good parsing library.

Does Idris have universe polymorphism? What is the type of Type?

Rather than universe polymorphism, Idris has a cumulative hierarchy of universes; Type : Type 1, Type 1 : Type 2, etc. Cumulativity means that if x : Type n and n <= m, then x : Type m. Universe levels are always inferred by Idris, and cannot be specified explicitly. The REPL command :type Type 1 will result in an error, as will attempting to specify the universe level of any type.

Why does Idris use Double instead of Float64?

Historically the C language and many other languages have used the names Float and Double to represent floating point numbers of size 32 and 64 respectively. Newer languages such as Rust and Julia have begun to follow the naming scheme described in IEEE Standard for Floating-Point Arithmetic (IEEE 754). This describes single and double precision numbers as Float32 and Float64; the size is described in the type name.

Due to developer familiarity with the older naming convention, and choice by the developers of Idris, Idris uses the C style convention. That is, the name Double is used to describe double precision numbers, and Idris does not support 32 bit floats at present.

What is -ffreestanding?

The freestanding flag is used to build Idris binaries which have their libs and compiler in a relative path. This is useful for building binaries where the install directory is unknown at build time. When passing this flag, the IDRIS_LIB_DIR environment variable needs to be set to the path where the Idris libs reside relative to the idris executable. The IDRIS_TOOLCHAIN_DIR environment variable is optional, if that is set, Idris will use that path to find the C compiler.


IDRIS_LIB_DIR="./libs" IDRIS_TOOLCHAIN_DIR="./mingw/bin" CABALFLAGS="-fffi -ffreestanding -frelease" make

What does the name ‘Idris’ mean?

British people of a certain age may be familiar with this singing dragon. If that doesn’t help, maybe you can invent a suitable acronym :-) .

Will there be support for Unicode characters for operators?

There are several reasons why we should not support Unicode operators:

  • It’s hard to type (this is important if you’re using someone else’s code, for example). Various editors have their own input methods, but you have to know what they are.
  • Not every piece of software easily supports it. Rendering issues have been noted on some mobile email clients, terminal-based IRC clients, web browsers, etc. There are ways to resolve these rendering issues but they provide a barrier to entry to using Idris.
  • Even if we leave it out of the standard library (which we will in any case!) as soon as people start using it in their library code, others have to deal with it.
  • Too many characters look too similar. We had enough trouble with confusion between 0 and O without worrying about all the different kinds of colons and brackets.
  • There seems to be a tendency to go over the top with use of Unicode. For example, using sharp and flat for delay and force (or is it the other way around?) in Agda seems gratuitous. We don’t want to encourage this sort of thing, when words are often better.

With care, Unicode operators can make things look pretty but so can lhs2TeX. Perhaps in a few years time things will be different and software will cope better and it will make sense to revisit this. For now, however, Idris will not be offering arbitrary Unicode symbols in operators.

This seems like an instance of Wadler’s Law in action.

This answer is based on Edwin Brady’s response in the following pull request.

Where can I find more answers?

There is an Unofficial FAQ on the wiki on GitHub which answers more technical questions and may be updated more often.