Sunday 20 September 2009

Real World Haskell

I finally got around to ordering Real World Haskell. Currently the only book on Haskell I have is The Haskell School of Expression which is enjoyable, but I've never got around to installing the relevant packages and passive reading only gets you so far!

I thought I'd try to keep notes as I go through the book as that seems to be a good way to prompt myself to actually use books (working through PAIP was very rewarding, so hopefully RWH will be a similar experience!) rather than just adorn my study with expensive door stops. This isn't meant to make particular sense, rather just a brain dump as I read through it.

So on with the book. Chapter 1 covers setting up the environment. There's a few interesting ghci commands I had not seen like :set prompt and :module + M. :set +t will always print the type of an expression. The "it" that is displayed is the name of a special variable which contains the value of the last expression evaluated. :info displays the precedence of operators. For example / has higher precedence than +.

Prelude> :info (+)
class (Eq a, Show a) => Num a where
(+) :: a -> a -> a
-- Defined in GHC.Num
infixl 6 +

Prelude> :info (/)
class (Num a) => Fractional a where
(/) :: a -> a -> a
-- Defined in GHC.Real
infixl 7 /

An infix operator can be converted to a prefix operator by enclosing the operator in parentheses. Similarly, back-ticks can convert a prefix operator into an infix operator.

Enumerations are a way of producing lists of numbers. The Haskell Cheat Sheet gives a great summary of the possibilities.

[1..3] -> [1,2,3]
[1..] -> [1,2,3,4,...] (infinite list)
[10..1] -> [] (ranges only go forward)
[0, -1 ..] -> [0, -1, -2, ...] (infinite list)
[1,3..10] -> [1,3,5,7,9] (list from 1 to 10 with diff of 2)

-- Also applies to any Enum class
['a' .. 'e'] -> "abcde"
['a', 'd' .. 'z'] -> "adgjmpsvy"

Don't use floating points for enumeration because the behaviour can be "quirky" (rounding errors and so on).

The empty string is a synonym for [] for the type Char,so 'a' : "" -> "a".

Rational numbers are defined in the Data.Ratio module, and constructed with % (e.g. 22 % 7 is nearly Pi).

The exercises showed me a function I'd not seen before. interact is a function which takes a function which takes stdin as input, and returns a string which is written to stdout (e.g. interact :: (String -> String) -> IO()). Also, once I grokked that "wc" was referring to the Unix command it all became a little clearer!