lenses-intro

Introduction to lenses in Haskell - examples of usage.

Command line

To compile and execute run:

$ stack build
$ stack exec lenses-intro-exe

or in one-liner:

$ stack build && stack exec lenses-intro-exe

REPL

To use in GHC REPL run:

$ stack ghci --only-main
λ main

The option --only-main imports only main module. You can skip it, but in that case when you try executing run function it will be ambiguous.

You can also run separate examples in ghci by:

λ Example1.run
...
λ Example2.run
... etc.

If you want to work in a scope of a specific module you can use for example:

λ :m +Example1
λ run

or simply:

λ import Example1
λ run

You can also "unimport" a specific module by:

λ :m -Example1

You can inspect your code in for example in following ways:

λ :t Point
Point :: a -> a -> Point a
λ :i Point
data Point a = Point {_x :: a, _y :: a}
instance Show a => Show (Point a)
λ :k Point
Point :: * -> *
λ :t x
x :: Functor f => (a -> f a) -> Point a -> f (Point a)
λ :i x
x :: Control.Lens.Type.Lens' (Point a) a

Where :t stands for type, :i for information and :k for kind.

You can reload the code after changes with :r and quit with :q.