The Idris REPL

Idris comes with a REPL.


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


Idris supports initialisation scripts.

Initialisation scripts

When the Idris REPL starts up, it will attempt to open the file repl/init in Idris’s application data directory. The application data directory is the result of the Haskell function call getAppUserDataDirectory "idris", which on most Unix-like systems will return $HOME/.idris and on various versions of Windows will return paths such as C:/Documents And Settings/user/Application Data/appName.

The file repl/init is a newline-separate list of REPL commands. Not all commands are supported in initialisation scripts — only the subset that will not interfere with the normal operation of the REPL. In particular, setting colours, display options such as showing implicits, and log levels are supported.

Example initialisation script

:colour prompt white italic bold
:colour implicit magenta italic

The REPL Commands

The current set of supported commands are:

Command Arguments Purpose
<expr>   Evaluate an expression
:t :type <expr> Check the type of an expression
:core <expr> View the core language representation of a term
:miss :missing <name> Show missing clauses
:doc <name> Show internal documentation
:mkdoc <namespace> Generate IdrisDoc for namespace(s) and dependencies
:apropos [<package list>] <name> Search names, types, and documentation
:s :search [<package list>] <expr> Search for values by type
:wc :whocalls <name> List the callers of some name
:cw :callswho <name> List the callees of some name
:browse <namespace> List the contents of some namespace
:total <name> Check the totality of a name
:r :reload   Reload current file
:l :load <filename> Load a new file
:cd <filename> Change working directory
:module <module> Import an extra module
:e :edit   Edit current file using $EDITOR or $VISUAL
:m :metavars   Show remaining proof obligations (holes)
:p :prove <hole> Prove a hole
:a :addproof <name> Add proof to source file
:rmproof <name> Remove proof from proof stack
:showproof <name> Show proof
:proofs   Show available proofs
:x <expr> Execute IO actions resulting from an expression using the interpreter
:c :compile <filename> Compile to an executable [codegen] <filename>
:exec :execute [<expr>] Compile to an executable and run
:dynamic <filename> Dynamically load a C library (similar to %dynamic)
:dynamic   List dynamically loaded C libraries
:? :h :help   Display this help text
:set <option> Set an option (errorcontext, showimplicits)
:unset <option> Unset an option
:color :colour <option> Turn REPL colours on or off; set a specific colour
:consolewidth auto|infinite|<number> Set the width of the console
:printerdepth <number-or-blank> Set the maximum pretty-printing depth, or infinite if nothing specified
:q :quit   Exit the Idris system
:w :warranty   Displays warranty information
:let (<top-level-declaration>)... Evaluate a declaration, such as a function definition, instance implementation, or fixity declaration
:unlet :undefine (<name>)... Remove the listed repl definitions, or all repl definitions if no names given
:printdef <name> Show the definition of a function
:pp :pprint <option> <number> <name> Pretty prints an Idris function in either LaTeX or HTML and for a specified width.

Using the REPL

Getting help

The command :help (or :h or :?) prints a short summary of the available commands.

Quitting Idris

If you would like to leave Idris, simply use :q or :quit.

Evaluating expressions

To evaluate an expression, simply type it. If Idris is unable to infer the type, it can be helpful to use the operator the to manually provide one, as Idris’s syntax does not allow for direct type annotations. Examples of the include:

Idris> the Nat 4
4 : Nat
Idris> the Int 4
4 : Int
Idris> the (List Nat) [1,2]
[1,2] : List Nat
Idris> the (Vect _ Nat) [1,2]
[1,2] : Vect 2 Nat

This may not work in cases where the expression still involves ambiguous names. The name can be disambiguated by using the with keyword:

Idris> sum [1,2,3]
When elaborating an application of function Prelude.Foldable.sum:
        Can't disambiguate name: Prelude.List.::,
Idris> with List sum [1,2,3]
6 : Integer

Adding let bindings

To add a let binding to the REPL, use :let. It’s likely you’ll also need to provide a type annotation. :let also works for other declarations as well, such as data.

Idris> :let x : String; x = "hello"
Idris> x
"hello" : String
Idris> :let data Foo : Type where Bar : Foo
Idris> Bar
Bar : Foo

Getting type information

To ask Idris for the type of some expression, use the :t command. Additionally, if used with an overloaded name, Idris will provide all overloadings and their types. To ask for the type of an infix operator, surround it in parentheses.

Idris> :t "foo"
"foo" : String
Idris> :t plus : Nat -> Nat -> Nat
Idris> :t (++)
Builtins.++ : String -> String -> String
Prelude.List.++ : (List a) -> (List a) -> List a
Prelude.Vect.++ : (Vect m a) -> (Vect n a) -> Vect (m + n) a
Idris> :t plus 4
plus (Builtins.fromInteger 4) : Nat -> Nat

You can also ask for basic information about typeclasses with :doc:

Idris> :doc Monad
Type class Monad


    (>>=) : Monad m => m a -> (a -> m b) -> m b

        infixl 5

    Monad id
    Monad PrimIO
    Monad IO
    Monad Maybe


Other documentation is also available from :doc:

Idris> :doc (+)
Prelude.Classes.+ : (a : Type) -> (Num a) -> a -> a -> a

infixl 8

        Class constraint Prelude.Classes.Num a
        __pi_arg : a
        __pi_arg1 : a
Idris> :doc Vect
Data type Prelude.Vect.Vect : Nat -> Type -> Type



Prelude.Vect.Nil : (a : Type) -> Vect 0 a

Prelude.Vect.:: : (a : Type) -> (n : Nat) -> a -> (Vect n a) -> Vect (S n) a

infixr 7

        Vect n a
Idris> :doc Monad
Type class Prelude.Monad.Monad

Prelude.Monad.>>= : (m : Type -> Type) -> (a : Type) -> (b : Type) -> (Monad m) -> (m a) -> (a -> m b) -> m b

infixl 5

        Class constraint Prelude.Monad.Monad m
        __pi_arg : m a
        __pi_arg1 : a -> m b

Finding things

The command :apropos searches names, types, and documentation for some string, and prints the results. For example:

Idris> :apropos eq
eqPtr : Ptr -> Ptr -> IO Bool

eqSucc : (left : Nat) -> (right : Nat) -> (left = right) -> S left = S right
S preserves equality

lemma_both_neq : ((x = x') -> _|_) -> ((y = y') -> _|_) -> ((x, y) = (x', y')) -> _|_

lemma_fst_neq_snd_eq : ((x = x') -> _|_) -> (y = y') -> ((x, y) = (x', y)) -> _|_

lemma_snd_neq : (x = x) -> ((y = y') -> _|_) -> ((x, y) = (x, y')) -> _|_

lemma_x_eq_xs_neq : (x = y) -> ((xs = ys) -> _|_) -> (x :: xs = y :: ys) -> _|_

lemma_x_neq_xs_eq : ((x = y) -> _|_) -> (xs = ys) -> (x :: xs = y :: ys) -> _|_

lemma_x_neq_xs_neq : ((x = y) -> _|_) -> ((xs = ys) -> _|_) -> (x :: xs = y :: ys) -> _|_

prim__eqB16 : Bits16 -> Bits16 -> Int

prim__eqB16x8 : Bits16x8 -> Bits16x8 -> Bits16x8

prim__eqB32 : Bits32 -> Bits32 -> Int

prim__eqB32x4 : Bits32x4 -> Bits32x4 -> Bits32x4

prim__eqB64 : Bits64 -> Bits64 -> Int

prim__eqB64x2 : Bits64x2 -> Bits64x2 -> Bits64x2

prim__eqB8 : Bits8 -> Bits8 -> Int

prim__eqB8x16 : Bits8x16 -> Bits8x16 -> Bits8x16

prim__eqBigInt : Integer -> Integer -> Int

prim__eqChar : Char -> Char -> Int

prim__eqFloat : Float -> Float -> Int

prim__eqInt : Int -> Int -> Int

prim__eqString : String -> String -> Int

prim__syntactic_eq : (a : Type) -> (b : Type) -> (x : a) -> (y : b) -> Maybe (x = y)

sequence : Traversable t => Applicative f => (t (f a)) -> f (t a)

sequence_ : Foldable t => Applicative f => (t (f a)) -> f ()

Eq : Type -> Type
The Eq class defines inequality and equality.

GTE : Nat -> Nat -> Type
Greater than or equal to

LTE : Nat -> Nat -> Type
Proofs that n is less than or equal to m

gte : Nat -> Nat -> Bool
Boolean test than one Nat is greater than or equal to another

lte : Nat -> Nat -> Bool
Boolean test than one Nat is less than or equal to another

ord : Char -> Int
Convert the number to its ASCII equivalent.

replace : (x = y) -> (P x) -> P y
Perform substitution in a term according to some equality.

sym : (l = r) -> r = l
Symmetry of propositional equality

trans : (a = b) -> (b = c) -> a = c
Transitivity of propositional equality

:search does a type-based search, in the spirit of Hoogle. See Type-directed search (:search) for more details. Here is an example:

Idris> :search a -> b -> a
= Prelude.Basics.const : a -> b -> a
Constant function. Ignores its second argument.

= assert_smaller : a -> b -> b
Assert to the totality checker than y is always structurally
smaller than x (which is typically a pattern argument)

> malloc : Int -> a -> a

> Prelude.pow : Num a => a -> Nat -> a

> Prelude.Classes.(*) : Num a => a -> a -> a

> Prelude.Classes.(+) : Num a => a -> a -> a
... (More results)

:search can also look for dependent types:

Idris> :search plus (S n) n = plus n (S n)
< Prelude.Nat.plusSuccRightSucc : (left : Nat) ->
                                  (right : Nat) ->
                                  S (left + right) = left + S right

Loading and reloading Idris code

The command :l File.idr will load File.idr into the currently-running REPL, and :r will reload the last file that was loaded.


All Idris definitions are checked for totality. The command :total <NAME> will display the result of that check. If a definition is not total, this may be due to an incomplete pattern match. If that is the case, :missing or :miss will display the missing cases.

Editing files

The command :e launches your default editor on the current module. After control returns to Idris, the file is reloaded.

Invoking the compiler

The current module can be compiled to an executable using the command :c <FILENAME> or :compile <FILENAME>. This command allows to specify codegen, so for example JavaScript can be generated using :c javascript <FILENAME>. The :exec command will compile the program to a temporary file and run the resulting executable.

IO actions

Unlike GHCI, the Idris REPL is not inside of an implicit IO monad. This means that a special command must be used to execute IO actions. :x tm will execute the IO action tm in an Idris interpreter.

Dynamically loading C libraries

Sometimes, an Idris program will depend on external libraries written in C. In order to use these libraries from the Idris interpreter, they must first be dynamically loaded. This is achieved through the %dynamic <LIB> directive in Idris source files or through the :dynamic <LIB> command at the REPL. The current set of dynamically loaded libraries can be viewed by executing :dynamic with no arguments. These libraries are available through the Idris FFI in type providers and :exec.


Idris terms are available in amazing colour! By default, the Idris REPL uses colour to distinguish between data constructors, types or type constructors, operators, bound variables, and implicit arguments. This feature is available on all POSIX-like systems, and there are plans to allow it to work on Windows as well.

If you do not like the default colours, they can be turned off using the command

:colour off

and, when boredom strikes, they can be re-enabled using the command

:colour on

To modify a colour, use the command


where <CATEGORY is one of keyword, boundvar, implicit, function, type, data, or prompt, and is a space-separated list drawn from the colours and the font options. The available colours are default, black, yellow, cyan, red, blue, white, green, and magenta. If more than one colour is specified, the last one takes precedence. The available options are dull and vivid, bold and nobold, italic and noitalic, underline and nounderline, forming pairs of opposites. The colour default refers to your terminal’s default colour.

The colours used at startup can be changed using REPL initialisation scripts.

Colour can be disabled at startup by the --nocolour command-line option.