An introduction to Elm

Elm

A delightful language for reliable webapps.

Generate JavaScript with great performance and no runtime exceptions.

http://elm-lang.org/

A gentle introduction to types

Most languages have types

Elm is statically typed

The type spectrum

#include <stdio.h>

int main ()
{
    int a = 2;
    char b[] = "2";

    printf("2 + 2 = %d", a + b);
}

add.c

$ gcc add.c -o add

#include <stdio.h>

int main ()
{
    int a = 2;
    char b[] = "2";

    printf("2 + 2 = %d", a + b);
}

add.c

$ gcc add.c -o add
add.c: In function ‘main’:
add.c:8:22: warning: format ‘%d’ expects argument of type ‘int’, but
            argument 2 has type ‘char *’ [-Wformat=]
     printf("2 + 2 = %d", a + b);

#include <stdio.h>

int main ()
{
    int a = 2;
    char b[] = "2";

    printf("2 + 2 = %d", a + b);
}

add.c

$ gcc add.c -o add
add.c: In function ‘main’:
add.c:8:22: warning: format ‘%d’ expects argument of type ‘int’, but
            argument 2 has type ‘char *’ [-Wformat=]
     printf("2 + 2 = %d", a + b);

$ ./add
2 + 2 = 1152074146

$ ./add
2 + 2 = 450267314

?????

public class Add {
    public static void main(String[] args) {
        int a = 2;
        String b = "2";

        System.out.printf("2 + 2 = %d", a + b);
    }
}

Add.java

$ javac Add.java

public class Add {
    public static void main(String[] args) {
        int a = 2;
        String b = "2";

        System.out.printf("2 + 2 = %d", a + b);
    }
}

Add.java

$ javac Add.java

$ java Add      
2 + 2 = Exception in thread "main" java.util.IllegalFormatConversionException: d != java.lang.String
	at java.util.Formatter$FormatSpecifier.failConversion(java.base@9-Ubuntu/Formatter.java:4275)
	at java.util.Formatter$FormatSpecifier.printInteger(java.base@9-Ubuntu/Formatter.java:2790)
	at java.util.Formatter$FormatSpecifier.print(java.base@9-Ubuntu/Formatter.java:2744)
	at java.util.Formatter.format(java.base@9-Ubuntu/Formatter.java:2525)
	at java.io.PrintStream.format(java.base@9-Ubuntu/PrintStream.java:974)
	at java.io.PrintStream.printf(java.base@9-Ubuntu/PrintStream.java:870)
	at Add.main(Add.java:6)

Runtime errors!

a = 2
b = "2"

main = putStrLn ("2 + 2 = " ++ show (a + b))

add.hs

$ ghc add.hs
[1 of 1] Compiling Main             ( add.hs, add.o )

add.hs:1:5:
    No instance for (Num [Char]) arising from the literal ‘2’
    In the expression: 2
    In an equation for ‘a’: a = 2

add.hs:4:40:
    No instance for (Num [Char]) arising from a use of ‘+’
    In the first argument of ‘show’, namely ‘(a + b)’
    In the second argument of ‘(++)’, namely ‘show (a + b)’
    In the first argument of ‘putStrLn’, namely
      ‘("2 + 2 = " ++ show (a + b))’

Compile time errors!

"Generate JavaScript with great performance and no runtime exceptions."

import Html exposing (text)

main =
  text "Hello, world!"

import Html exposing (text)
import List exposing (..)

greetings =
  [ "Hola, mundo!"
  , "Hello, world!"
  , "Bonjour, le monde!"
  ]

main =
  text (head greetings)

import Html exposing (text)
import List exposing (..)

greetings =
  [ "Hola, mundo!"
  , "Hello, world!"
  , "Bonjour, le monde!"
  ]

main =
  text (head greetings)

import Html exposing (text)
import List exposing (..)

greetings =
  [ "Hola, mundo!"
  , "Hello, world!"
  , "Bonjour, le monde!"
  ]

greeting =
  case head greetings of
    Just value -> value
    Nothing -> "No greetings! :("

main =
  text greeting