Smart constructors
https://wiki.haskell.org/Smart_constructors
智能构造器是指为了在构造过程中对构造器的参数加以限制，不提供缺省的数据构造器，而是提供特别的函数来充当构造器。
智能构造器可分为运行期检查和编译期检查两类。
示例
{-# LANGUAGE FlexibleInstances #-}
module Resistor (
Resistor, -- abstract, hiding constructors
metalResistor1, -- only way to build a metal resistor
metalResistor2, -- only way to build a metal resistor
Resistor3, -- abstract, hiding constructors
resistor3, -- only way to build a metal resistor
) where
import Control.Exception(assert)
-- runtime check
data Resistor = Metal Bands
| Ceramic Bands
deriving Show
type Bands = Int
metalResistor1 :: Bands -> Resistor
metalResistor1 n | n < 4 || n > 8 = error "Invalid number of resistor bands"
| otherwise = Metal n
metalResistor2 :: Bands -> Resistor
metalResistor2 n = assert (n >= 4 && n <= 8) $ Metal n
-- compile-time check
data Z = Z
data S a = S a
class Card c where
instance Card Z where
instance (Card c) => Card (S c) where
class Card size => InBounds size where
instance InBounds (S (S (S (S Z)))) where -- four
instance InBounds (S (S (S (S (S Z))))) where -- five
instance InBounds (S (S (S (S (S (S Z)))))) where -- six
instance InBounds (S (S (S (S (S (S (S Z))))))) where -- seven
instance InBounds (S (S (S (S (S (S (S (S Z)))))))) where -- eight
d0 = undefined :: Z
d3 = undefined :: S (S (S Z))
d4 = undefined :: S (S (S (S Z)))
d6 = undefined :: S (S (S (S (S (S Z)))))
d8 = undefined :: S (S (S (S (S (S (S (S Z)))))))
d10 = undefined :: S (S (S (S (S (S (S (S (S (S Z)))))))))
data Resistor3 size = Resistor3 deriving Show
resistor3 :: InBounds size => size -> Resistor3 size
resistor3 _ = Resistor3
运行期检查
*Resistor> metalResistor1 4
Metal 4
*Resistor> metalResistor2 7
Metal 7
*Resistor> metalResistor1 9
*** Exception: Invalid number of resistor bands
CallStack (from HasCallStack):
error, called at smart_constructors.hs:18:37 in main:Resistor
*Resistor> metalResistor2 0
*** Exception: Assertion failed
CallStack (from HasCallStack):
assert, called at smart_constructors.hs:22:20 in main:Resistor
编译期检查
*Resistor> resistor3 d0
<interactive>:15:1: error:
• No instance for (InBounds Z) arising from a use of ‘resistor3’
• In the expression: resistor3 d0
In an equation for ‘it’: it = resistor3 d0
*Resistor> resistor3 d3
<interactive>:16:1: error:
• No instance for (InBounds (S (S (S Z))))
arising from a use of ‘resistor3’
• In the expression: resistor3 d3
In an equation for ‘it’: it = resistor3 d3
*Resistor> resistor3 d4
Resistor3
*Resistor> :t resistor3 d4
resistor3 d4 :: Resistor3 (S (S (S (S Z))))
*Resistor> resistor3 d6
Resistor3
*Resistor> resistor3 d8
Resistor3
*Resistor> :t resistor3 d8
resistor3 d8 :: Resistor3 (S (S (S (S (S (S (S (S Z))))))))
*Resistor> resistor3 d10
<interactive>:22:1: error:
• No instance for (InBounds
(S (S (S (S (S (S (S (S (S (S Z)))))))))))
arising from a use of ‘resistor3’
• In the expression: resistor3 d10