167 lines
3.3 KiB
Haskell
167 lines
3.3 KiB
Haskell
-----------------------------------------------------------
|
|
-- If Else and Guards
|
|
-----------------------------------------------------------
|
|
|
|
{- IfElse
|
|
* If-else expressions denote "normal" values
|
|
-}
|
|
three :: Integer
|
|
three = if True then 3 else 4
|
|
|
|
four :: Integer
|
|
four = if False then 0 else 4
|
|
|
|
collatzNext :: Integer -> Integer
|
|
collatzNext x =
|
|
if x `mod` 2 == 0 then
|
|
x `div` 2
|
|
else
|
|
3*x+1
|
|
|
|
{- Nesting
|
|
If-else expressions with multiple cases are possible
|
|
with nesting.
|
|
-}
|
|
describe :: Integer -> String
|
|
describe x =
|
|
if x < 3 then
|
|
"small"
|
|
else if x < 5 then
|
|
"medium"
|
|
else
|
|
"large"
|
|
|
|
{- Guards
|
|
An alternative way to declare functions by case analysis
|
|
are guards.
|
|
-}
|
|
describe' :: Integer -> String
|
|
describe' x
|
|
| x < 3 = "small"
|
|
| x < 4 = "medium"
|
|
| otherwise = "large"
|
|
|
|
sign :: Integer -> Integer
|
|
sign x
|
|
| x < 0 = -1
|
|
| x > 0 = 1
|
|
| otherwise = 0
|
|
|
|
{- Exercise
|
|
rewrite the function 'fIfElse' using guards.
|
|
-}
|
|
fIfElse :: Integer -> String
|
|
fIfElse n =
|
|
if n `mod` 2 == 1 then
|
|
if n `mod` 3 /= 0 then
|
|
"Oddity"
|
|
else
|
|
"Odd"
|
|
else
|
|
"Even"
|
|
|
|
|
|
fGuard :: Integer -> String
|
|
fGuard = error "fixme"
|
|
|
|
|
|
-----------------------------------------------------------
|
|
-- Cases and pattern matching
|
|
-----------------------------------------------------------
|
|
|
|
{- Cases 1
|
|
Aside from if-else and guards, Haskell also allows for
|
|
functions to be declared in separate clauses.
|
|
-}
|
|
|
|
count :: Integer -> String
|
|
count 0 = "zero"
|
|
count 1 = "one"
|
|
count 2 = "two"
|
|
count _ = "I don't know"
|
|
|
|
{- Cases 2
|
|
There is also an alternative syntax for cases (that make
|
|
declarations a bit easier to maintain in some cases).
|
|
-}
|
|
|
|
count' :: Integer -> String
|
|
count' n = case n of
|
|
0 -> "zero"
|
|
1 -> "one"
|
|
2 -> "two"
|
|
_ -> "I don't know"
|
|
|
|
|
|
{- Cases Importance
|
|
The true "power" of cases is in conjunction with custom
|
|
types and pattern matching. We will learn how this works
|
|
later.
|
|
-}
|
|
|
|
data Shape
|
|
= Rectangle Float Float
|
|
| Circle Float
|
|
|
|
circumference :: Shape -> Float
|
|
circumference shape = case shape of
|
|
Rectangle length width ->
|
|
2*length + 2*width
|
|
Circle radius ->
|
|
2*radius*pi
|
|
|
|
{- Exercise
|
|
Define a function
|
|
'area :: Shape -> Float'
|
|
to compute the area of any given shape. Use spearate
|
|
cases such as in 'Case 1' above.
|
|
-}
|
|
area :: Shape -> Float
|
|
area = error "fixme"
|
|
|
|
|
|
-----------------------------------------------------------
|
|
-- Let and where
|
|
-----------------------------------------------------------
|
|
|
|
{- Let Bindings
|
|
It is possible to name one or more values in a
|
|
"let-block" and these abbreviations in the subsequent
|
|
expression.
|
|
-}
|
|
five :: Integer
|
|
five =
|
|
let
|
|
x = 2
|
|
y = x + 1
|
|
in
|
|
x + y
|
|
|
|
myFunc :: Integer -> Integer
|
|
myFunc x =
|
|
let
|
|
complicated =
|
|
x `mod` 2 == 0 && x `mod` 4 /= 0
|
|
in
|
|
if complicated then
|
|
x `div` 2
|
|
else
|
|
x + 1
|
|
|
|
{- Where
|
|
Where is the same as let, but it does not preceed but
|
|
follow a "main" declaration.
|
|
-}
|
|
six :: Integer
|
|
six =
|
|
x + y
|
|
where
|
|
x = 2
|
|
y = x + 2
|
|
|
|
myFunc' :: Integer -> Integer
|
|
myFunc' x =
|
|
(magicNumber * x) + 1
|
|
where
|
|
magicNumber = x + 42
|