Examples are mostly adapted from the Idris tutorial.
Source File Structure¶
Source files consist of:
- An optional Module Header.
- Zero or more Imports.
- Zero or more declarations, e.g. Variables, Data types, etc.
module MyModule -- module header import Data.Vect -- an import %default total -- a directive foo : Nat -- a declaration foo = 5
A file can start with a module header, introduced by the
Module names can be hierarchical, with parts separated by
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
import makes the names in another module available for use by the current
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 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.
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
data Vect : Nat -> Type -> Type where Nil : Vect Z a (::) : (x : a) -> (xs : Vect n a) -> Vect (S n) a
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 <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 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
Arguments in anonymous functions are separated by comma.
(\x => <expr>) (\x, y => <expr>)
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
Qualified names can be written using two different shorthands:
- Names in modules that are imported using an alias can be qualified by the alias.
- The name can be qualified by the shortest unique suffix of the
namespace in question. For example, the
lengthcase above can likely be shortened to
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>