Syntax Guide

Examples are mostly adapted from the Idris tutorial.

Source File Structure

Source files consist of:

  1. An optional Module Header.
  2. Zero or more Imports.
  3. Zero or more declarations, e.g. Variables, Data types, etc.

For example:

module MyModule   -- module header

import Data.Vect  -- an import

%default total    -- a directive

foo : Nat         -- a declaration
foo = 5

Module Header

A file can start with a module header, introduced by the module keyword:

module Semantics

Module names can be hierarchical, with parts separated by .:

module Semantics.Transform

Each file can define only a single module, which includes everything defined in that file.

Like with declarations, a docstring can be used to provide documentation for a module:

||| Implementation of predicate transformer semantics.
module Semantics.Transform


An import makes the names in another module available for use by the current module:

import Data.Vect

All the declarations in an imported module are available for use in the file. In a case where a name is ambiguous — e.g. because it is imported from multiple modules, or appears in multiple visible namespaces — the ambiguity can be resolved using Qualified Names. (Often, the compiler can resolve the ambiguity for you, using the types involved.)

Imported modules can be given aliases to make qualified names more compact:

import Data.Vect as V

Note that names made visible by import are not, by default, re-exported to users of the module being written. This can be done using import public:

import public Data.Vect


A variable is always defined by defining its type on one line, and its value on the next line, using the syntax

<id> : <type>
<id> = <value>


x : Int
x = 100
hello : String
hello = "hello"


In Idris, types are first class values. So a type declaration is the same as just declaration of a variable whose type is Type. In Idris, variables that denote a type need not be capitalised. Example:

MyIntType : Type
MyIntType = Int

a more interesting example:

MyListType : Type
MyListType = List Int

While capitalising types is not required, the rules for generating implicit arguments mean it is often a good idea.

Data types

Idris provides two kinds of syntax for defining data types. The first, Haskell style syntax, defines a regular algebraic data type. For example

data Either a b = Left a | Right b


data List a = Nil | (::) a (List a)

The second, more general kind of data type, is defined using Agda or GADT style syntax. This syntax defines a data type that is parameterised by some values (in the Vect example, a value of type Nat and a value of type Type).

data Vect : Nat -> Type -> Type where
  Nil  : Vect Z a
  (::) : (x : a) -> (xs : Vect n a) -> Vect (S n) a

The signature of type constructors may use dependent types

data DPair : (a : Type) -> (a -> Type) -> Type where
  MkDPair : {P : a -> Type} -> (x : a) -> (pf : P x) -> DPair a P


There is a special syntax for data types with one constructors and multiple fields.

record A a where
  constructor MkA
  foo, bar : a
  baz : Nat

This defines a constructor as well as getter and setter function for each field.

MkA : a -> a -> Nat -> A a
foo : A a -> a
set_foo : a -> A a -> A a

The types of record fields may depend on the value of other fields

record Collection a where
  constructor MkCollection
  size : Nat
  items : Vect size a

Setter functions are only provided for fields that do not use dependant types. In the example above neither set_size nor set_items are defined.


Inifinite data structures can be introduced with the codata keyword.

codata Stream : Type -> Type where
  (::) a -> Stream a -> Stream a

This is syntactic sugar for the following, which is usually preferred:

data Stream : Type -> Type where
  (::) a -> Inf (Stream a) -> Stream a

Every occurence of the the defined type in a constructor argument will be wrapped in the Inf type constructor. This has the effect of delaying the evaluation of the second argument when the data constructor is applied. An Inf argument is constructed using Delay (which Idris will insert implicitly) adn evaluated using Force (again inserted implicitly).

Furthermore, recursive calls under a Delay must be guarded by a constructor to pass the totality checker.



x + y
x - y
x * y
x / y
(x * y) + (a / b)

Equality and Relational

x == y
x /= y
x >= y
x > y
x <= y
x < y


x && y
x || y
not x


If Then Else

if <test> then <true> else <false>

Case Expressions

case <test> of
    <case 1>  => <expr>
    <case 2>  => <expr>
    otherwise => <expr>



Named functions are defined in the same way as variables, with the type followed by the definition.

<id> : <argument type> -> <return type>
<id> arg = <expr>


plusOne : Int -> Int
plusOne x = x + 1

Functions can also have multiple inputs, for example

makeHello : String -> String -> String
makeHello first last = "hello, my name is " ++ first ++ " " ++ last

Functions can also have named arguments. This is required if you want to annotate parameters in a docstring. The following shows the same makeHello function as above, but with named parameters which are also annotated in the docstring

||| Makes a string introducing a person
||| @first The person's first name
||| @last The person's last name
makeHello : (first : String) -> (last : String) -> String
makeHello first last = "hello, my name is " ++ first ++ " " ++ last

Like Haskell, Idris functions can be defined by pattern matching. For example

sum : List Int -> Int
sum []        = 0
sum (x :: xs) = x + (sum xs)

Similarly case analysis looks like

answerString : Bool -> String
answerString False = "Wrong answer"
answerString True = "Correct answer"

Dependent Functions

Dependent functions are functions where the type of the return value depends on the input value. In order to define a dependent function, named parameters must be used, since the parameter will appear in the return type. For example, consider

zeros : (n : Nat) -> Vect n Int
zeros Z     = []
zeros (S k) = 0 :: (zeros k)

In this example, the return type is Vect n Int which is an expression which depends on the input parameter n.


Arguments in anonymous functions are separated by comma.

(\x => <expr>)
(\x, y => <expr>)



public export




%specialise [<name list>]


Qualified Names

If multiple declarations with the same name are visible, using the name can result in an ambiguous situation. The compiler will attempt to resolve the ambiguity using the types involved. If it’s unable — for example, because the declarations with the same name also have the same type signatures — the situation can be cleared up using a qualified name.

A qualified name has the symbol’s namespace prefixed, separated by a .:


This would specifically reference a length declaration from Data.Vect.

Qualified names can be written using two different shorthands:

  1. Names in modules that are imported using an alias can be qualified by the alias.
  2. The name can be qualified by the shortest unique suffix of the namespace in question. For example, the length case above can likely be shortened to Vect.length.


-- Single Line
{- Multiline -}
||| Docstring (goes before definition)

Multi line String literals

foo = """
this is a
string literal"""


%lib <path>
%link <path>
%flag <path>
%include <path>
%hide <function>
%freeze <name>
%access <accessibility>
%default <totality>
%logging <level 0--11>
%dynamic <list of libs>
%name <list of names>
%error_handlers <list of names>
%language <extension>