haskell.md

Haskell

• using stack to compile/manage

• type stack ghci to get interactive session

• With emacs:

  • use haskell-mode; install with melpa as explained in the ReadMe file for haskell-mode

  • use inf-haskell for interactive session: put (require 'inf-haskell) in .emacs and then, with a .hs haskell file loaded in a buffer, use C-c C-l to get interactive session with that module automatically loaded.

  • At command line, used stack install intero; added the emacs melpa package intero, and then added the following to my ~/.emacs file:

    (add-hook 'haskell-mode-hook 'intero-mode)
    (defun haskell-mode-setup ()
      (setq haskell-process-type 'stack-ghci))
    (add-hook 'haskell-mode-hook 'interactive-haskell-mode)
    (add-hook 'haskell-mode-hook 'haskell-mode-setup)
    (require 'inf-haskell)

• Use :l myfile.hs to load a file

• Use :t object to get the type of an object

---

-- comments follow two hyphens
6 /= 3   -- not equal

-- define a function
doubleMe x = x + x

-- string = list of characters
['c', 'a', 't'] == "cat"

-- : operator to add to beginning of list
'c' : ['a', 't']

-- ++ operator to join two lists
['c'] ++ ['a', 't']

-- useful functions on lists
x = "karl has a cat"
head x
tail x
last x
init x
length x
null x
reverse x
take 5 x
drop 5 x
maximum x
minimum x
sum [3..10] -- integers 3 to 10
product [8,10..20] -- even numbers 8 to 20
'k' `elem` x -- contained in?

-- list comprehensions
[t*3 | t <- [5..10]]
[t*3 | t <- [5..14], t `mod` 3 == 0]

Recursive function example, using "patterns"; return -999 if input is negative.

fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = if n < 0
  then error "n should be non-negative"
  else (fib (n-1)) + (fib (n-2))