let式

http://haskell.g.hatena.ne.jp/hyuki/20060611/let

ghciで変数束縛したり関数束縛するときはletが便利だと今日気付く。
（最後に評価した値が変数itに束縛されるのも便利）

*Main> [['a'..'z'],['A'..'Z']]
["abcdefghijklmnopqrstuvwxyz","ABCDEFGHIJKLMNOPQRSTUVWXYZ"]
*Main> transpose it
["aA","bB","cC","dD","eE","fF","gG","hH","iI","jJ","kK","lL","mM","nN","oO","pP","qQ","rR","sS","tT","uU","vV","wW","xX","yY","zZ"]
*Main> concat it
"aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ"
*Main> let x = 12
*Main> x
12
*Main> x + 3
15
*Main> let fn n = n * 3
*Main> :t fn
fn :: (Num a) => a -> a
*Main> fn 4
12
*Main> :show bindings
x
fn
it
*Main>