Question

I cannot understand what is this “instance” means in Haskell (in line 19) :

16 > type Prog = [Function]
17 > data Function = Defun String String Exp
18 > --                deriving Show
19 > instance Show Function where
20 >   show (Defun n p e) = "\n" ++ n ++ "(" ++ p ++ ") { return " ++ show e ++ "; }\n"
21 >   showList [] = showString ""
22 >   showList (f:fs) = shows f . showl fs
23 >       where showl [] = showString ""
24 >             showl (f:fs) = shows f . showl fs

thx.

Answer 1

Read about type classes.

Show is a type class and you are defining a Show instance for Function datatype, so you can do something like

show (Defun "a" "b" someExpr)

and it will convert this to a string representation using the show definition for Function.
If you are from OO background then you can think like overloading the show function for Function data type.