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Pattern Matching
fun eval e =
case e of
Constant i => i
| Negate e2 => ~ (eval e2)
| Add(e1,e2) => (eval e1) + (eval e2)
| Multiply(e1,e2) => (eval e1) * (eval e2)
fun eval(Constant(i)) = i
| eval(Negate(e)) = ~ (eval e)
| eval(Add(e1, e2)) = (eval e1)+(eval e2)
| eval(Multiply(e1, e2)) = (eval e1)*(eval e2)Nested patterns
• We can nest patterns as deep as we want
– Just like we can nest expressions as deep as we want
– Often avoids hard-to-read, wordy nested case expressions
• So the full meaning of pattern-matching is to compare a pattern against a value for the “same shape” and bind variables to the “right parts”
Exceptions
예외처리는 raise으로 한다.
fun calc (n) = if n < 0.0
then raise Domain
else n * 1.04
val balance = calc(~10.0)
uncaught exception Domain [domain error]
raised at: test.sml:47.27-47.33
예외 핸들링은 handling으로 한다.
fun calc (n) = if n < 0.0
then raise Domain
else n * 1.04
val balance = calc(~10.0)
handle Domain => ~10.0
val calc = fn : real -> real
val balance = ~10.0 : realTail call
tail-recursive는 매우 효과적인 재귀 함수이다.
fun sum xs =
case xs of
[] => 0
| x::xs' => x + sum xs'
fun sum xs =
let fun aux(xs,acc) =
case xs of
[] => acc
| x::xs' => aux(xs',x+acc)
in
aux(xs,0)
end
fun rev xs =
case xs of
[] => []
| x::xs' => (rev xs') @ [x]
fun rev xs =
let fun aux(xs,acc) =
case xs of
[] => acc
| x::xs' => aux(xs',x::acc)
in
aux(xs,[])
end
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