Types
Types classify programs based on the values that they can compute. Types serve a number of roles in a program:

They allow the compiler to make decisions about the inmemory representation of a value.

They help programmers to communicate their intent to others, serving as a lightweight specification for the inputs and outputs of a function that the compiler can ensure the program adheres to.

They prevent various potential mistakes, such as adding a number to a string, and thus reduce the number of tests that are necessary for a program.

They help the Lean compiler automate the production of auxiliary code that can save boilerplate.
Lean's type system is unusually expressive. Types can encode strong specifications like "this sorting function returns a permutation of its input" and flexible specifications like "this function has different return types, depending on the value of its argument". The type system can even be used as a fullblown logic for proving mathematical theorems. This cuttingedge expressive power doesn't obviate the need for simpler types, however, and understanding these simpler types is a prerequisite for using the more advanced features.
Every program in Lean must have a type. In particular, every expression must have a type before it can be evaluated. In the examples so far, Lean has been able to discover a type on its own, but it is sometimes necessary to provide one. This is done using the colon operator:
#eval (1 + 2 : Nat)
Here, Nat
is the type of natural numbers, which are arbitraryprecision unsigned integers.
In Lean, Nat
is the default type for nonnegative integer literals.
This default type is not always the best choice.
In C, unsigned integers underflow to the largest representable numbers when subtraction would otherwise yield a result less than zero.
Nat
, however, can represent arbitrarilylarge unsigned numbers, so there is no largest number to underflow to.
Thus, subtraction on Nat
returns 0
when the answer would have otherwise been negative.
For instance,
#eval 1  2
evaluates to 0
rather
than 1
. To use a type that can represent the negative integers,
provide it directly:
#eval (1  2 : Int)
With this type, the result is 1
, as expected.
To check the type of an expression without evaluating it, use #check
instead of #eval
. For instance:
#check (1  2 : Int)
reports 1  2 : Int
without actually performing the subtraction.
When a program can't be given a type, an error is returned from both
#check
and #eval
. For instance:
#check String.append "hello" [" ", "world"]
outputs
application type mismatch
String.append "hello" [" ", "world"]
argument
[" ", "world"]
has type
List String : Type
but is expected to have type
String : Type
because the second argument to String.append
is expected to be a
string, but a list of strings was provided instead.