1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
|
{- Environments.
Uses lists for simplicity and to make the semantics clear.
A real implementation should use balanced trees or hash tables.
-}
module Env (Env,
eempty,
elookup,
eextend,
edomain,
efromlist,
efilter,
eremove)
where
import List
data Env a b = Env [(a,b)]
deriving (Show)
eempty :: Env a b
eempty = Env []
{- In case of duplicates, returns most recently added entry. -}
elookup :: (Eq a) => Env a b -> a -> Maybe b
elookup (Env l) k = lookup k l
{- May hide existing entries. -}
eextend :: Env a b -> (a,b) -> Env a b
eextend (Env l) (k,d) = Env ((k,d):l)
edomain :: (Eq a) => Env a b -> [a]
edomain (Env l) = nub (map fst l)
{- In case of duplicates, first entry hides others. -}
efromlist :: [(a,b)] -> Env a b
efromlist l = Env l
eremove :: (Eq a) => Env a b -> a -> Env a b
eremove (Env l) k = Env (filter ((/= k).fst) l)
efilter :: Env a b -> (a -> Bool) -> Env a b
efilter (Env l) p = Env (filter (p.fst) l)
|
