Geeking out with Lisp Flavoured Erlang

One of the features I dislike the most about Erlang is records. They're ugly and they require too much typing. Erlang makes me write


Dog = #dog{name = "Lolo", parent = #dog{name = "Max"}},
Name = Dog#dog.name,
ParentName = (Dog#dog.parent)#dog.name,
Dog1 = Dog#dog{
name = "Lola",
parent = (Dog#dog.parent)#dog{name = "Rocky"}}


When I want to write


Dog = #Dog{name = "Lolo", parent = #dog{name = "Max"}},
Name = Dog.name,
Size = Dog.size,
Dog1 = Dog{
name = "Lola",
parent = Dog.parent{name = "Rocky"}}


In the defense of records, they're just syntactic sugar over tuples and as such they enable fast access into a tuple's properties despite Erlang's dynamic nature. In compile time, they're converted into fast element() and setelement() calls that don't require looking up the property's index in the tuple. Still, I dislike them because 1) in many cases, I'd rather optimize for more concise code than faster execution and 2) if the Erlang compiler were smarter could allow us to write the more concise code above by inferring the types of variables in your program when possible. (I wrote a rant about this a long time ago, with a proposed solution to it in the form of a yet unfinished parse transform called Recless.)

Parameterized modules provide a somewhat more elegant and dynamic dealing with types, but they still require you to do too much work. You can define a parameterized module like this:

-module(dog, [Name, Size]).

This creates a single function called 'new/2' in the module 'dog'. You can call it as follows:


Dog = dog:new("Lola", "big").


It returns a tuple of the form


{dog, "Lola", "Big"}.


You can't set only a subset of the record's properties in the 'new' function. This doesn't work


Dog = dog:new("Lola").


To access the record's properties you need to define your own getter functions, e.g.


name() ->
Name.


which you can call it as follows:


Name = Dog:name().


(This involves a runtime lookup of the 'name' function in the 'dog' module, which is slower than a record accessor).

There's no way to set a record's properties after it's created short of altering the tuple directly with setelement(), which is lame and also quite brittle. To create a setter, you need do the following:


name(Val) ->
setelement(2, THIS, Val).


Then you can call


Dog1 = Dog:name("Lola").


to change the object's name.

When LFE came out I was hoping it would provide a better way of dealing with the problem. Unfortunately, records in LFE are quite similar to Erlang records, though they are a bit nicer. Instead of adding syntactic sugar, LFE creates a bunch of macros you can use to create a record and access its properties.


(record dog (name))
(let* ((dog (make-dog (name "Lola")))
(name (dog-name dog))
(dog1 (set-dog-name dog "Lolo")))
;; do something)


LFE still requires us to do too much typing when dealing with records to my taste, but LFE does give us a powerful tool to come up with our own solution to the problem: macros. We can use macros generate all those repetitive brittle parameterized module getters and setters that in vanilla Erlang we have to write by hand. This can help us avoid much the tedium involved in working with parameterized modules.

(ErlyWeb performs similar code generation on database modules, but it does direct manipulation of Erlang ASTs, which are gnarly.)

Let's start with the 'new' functions. We want to generate a bunch of 'new' functions allow us to set only a subset of the record's properties, implicitly setting the rest to 'undefined'.


(defun make_constructors (name props)
(let* (((props_acc1 constructors_acc1)
(: lists foldl
(match-lambda
((prop (props_acc constructors_acc))
(let* ((params (: lists reverse props_acc))
(constructor
`(defun new ,params
(new ,@params 'undefined))))
(list
(cons prop props_acc)
(cons constructor constructors_acc)))))
(list () ())
props))
(main_constructor
`(defun new ,props (tuple (quote ,name) ,@props))))
(: lists reverse (cons main_constructor constructors_acc1))))


This function take the module name and a list of properties. It returns a list of sexps of the form


(defun new (prop1 prop2 ...prop_n-m)
(new prop1 prop2 ... prop_n-m 'undefined))


as well as one sexp of the form


(defun (prop1 prop2 ... prop_n)
(tuple module_name prop1 prop2... prop_n)


The first set of 'new' functions successively call the next 'new' function in the chain, passing into it their list of parameters together with 'undefined' as the last parameter. The last 'new' function takes all the parameters needed to instantiate an object and returns a tuple whose first element is the module, and the rest are the object's property values. (We need to store the module name in the first element so we can use the parameterized modules calling convention, as you'll see later.)

Now let's create a function that generates the getters and setters


(defun make_accessors (props)
(let*
(((accessors idx1)
(: lists foldl
(match-lambda
((prop (acc idx))
(let* ((getter `(defun ,prop (obj) (element ,idx obj)))
(setter `(defun ,prop (val obj)
(setelement ,idx obj val))))
(list (: lists append
(list getter setter)
acc)
(+ idx 1)))))
(list () 2)
props)))
accessors)))


This function takes a list of properties and returns a list of sexps that implement the getters and setters for the module. Each getter takes the object and returns its (n + 1)th element, where n is the position of its property (the first element is reserved for the module name). Each setter takes the new value and the object and returns the tuple after setting its (n + 1)th element to the new value.

Now, let's tie it this up with the module declaration. We need to create a macro that generates the module declaration, constructors, getters, and setters, all in one swoop. But first, we need to expose make_constructors and make_accessors to the macro by nesting them inside a (eval-when-compile) sexp.


(eval-when-compile
(defun make_constructors ...)
(defun make_accessors ...))

(defmacro defclass
(((name . props) . modifiers)
(let* ((constructors (make_constructors name props))
(accessors (make_accessors props)))
`(progn
(defmodule ,name ,@modifiers)
,@constructors
,@accessors))))


(defclass returns a `(progn) sexp with a list of new macros that it defines. This is a trick Robert Virding taught me for creating a macro that in itself creates multiple macros.)


Now, let's see how this thing works. Create the file dog.lfe with the following code


;; import the macros here

(defclass (dog name size)
(export all))


Compile it from the shell:


(c 'dog)


Create a dog with no properties


> (: dog new)
#(dog undefined undefined)


Create a new dog with just a name


> (: dog new '"Lola")
#(dog "Lola" undefined)


Create a new dog with a name and a size


> (: dog new '"Lola" '"medium")
#(dog "Lola" "medium")


Get and set the dog's properties


> (let* ((dog (: dog new))
(dog1 (call dog 'name '"Lola"))
(dog2 (call dog1 'size '"medium")))
(list (call dog2 'name) (call dog2 'size)))
("Lola" "medium")


This code uses the same parameterized module function calling mechanism that allows us to pass a tuple instead of a function as the first parameter to the 'call' function. Erlang infers the name of the module that contains the function from the first element of the tuple.

As you can see, LFE is pretty powerful. I won't deny that sometimes I get the feeling of being lost in a sea of parenthesis, but over time the parentheses have grown on me. The benefits of macros speak for themselves. They give you great flexibility to change the language as you see fit.