Изучай Haskell ради Добра! Начало

Learn you a Haskell for Great Good:chapter - Starting-out

Starting Out

Ready, set, go!

Alright, let's get started! If you're the sort of horrible person who doesn't read introductions to things and you skipped it, you might want to read the last section in the introduction anyway because it explains what you need to follow this tutorial and how we're going to load functions.

The first thing we're going to do is run ghc's interactive mode and call some function to get a very basic feel for haskell. Open your terminal and type in ghci. You will be greeted with something like this.

GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help

Loading package base ... linking ... done.

Prelude>

Congratulations, you're in GHCI! The prompt here is Prelude> but because it can get longer when you load stuff into the session, we're going to use ghci>. If you want to have the same prompt, just type in :set prompt "ghci> ".

Here's some simple arithmetic.

ghci> 2 + 15

17

ghci> 49 * 100

4900

ghci> 1892 - 1472

420

ghci> 5 / 2

2.5

ghci>

This is pretty self-explanatory. We can also use several operators on one line and all the usual precedence rules are obeyed. We can use parentheses to make the precedence explicit or to change it.

ghci> (50 * 100) - 4999

1

ghci> 50 * 100 - 4999

1

ghci> 50 * (100 - 4999)

-244950

Pretty cool, huh? Yeah, I know it's not but bear with me. A little pitfall to watch out for here is negating numbers. If we want to have a negative number, it's always best to surround it with parentheses. Doing 5 * -3 will make GHCI yell at you but doing 5 * (-3) will work just fine.

Boolean algebra is also pretty straightforward. As you probably know, && means a boolean and, || means a boolean or. not negates a True or a False.

ghci> True && False

False

ghci> True && True

True

ghci> False || True

True

ghci> not False

True

ghci> not (True && True)

False

Testing for equality is done like so.

ghci> 5 == 5

True

ghci> 1 == 0

False

ghci> 5 /= 5

False