问题
I am fresh on SML and doing a homework by that. "Write a function number_in_month that takes a list of dates and a month (i.e., an int) and returns how many dates in the list are in the given month."
That's what I worked out and cannot see anything wrong with it. Please help.
`
fun number_in_month (dates: (int*int*int) list,month:int) =
if ((#2 (hd dates)) = month)
then val flag=1 flag+number_in_month(tl dates, month)
else number_in_month((tl dates),month)`
REPL tells that: replacing VAL with EQUALOP.
回答1:
You can't bind variables "that way". A binding of a variable is a declaration and thus cannot be done where an expression is expected.
In this case you have to use a let-in-end expression
fun foo x =
let
val a = 42
in
a*x
end
回答2:
Your problem is endless recursion. Compiler can't get out of it because independ of result if..then..else you're running your function again Try this:
fun number_in_month (dates: (int*int*int) list,month:int) =
if null dates
then 0
else if ((#2 (hd dates)) = month)
then val flag=1 flag+number_in_month(tl dates, month)
else number_in_month((tl dates),month)
回答3:
I tried to fix it by myself and that was my solution:
fun number_in_month (dias: (int*int*int) list,mes:int) = if null dias then 0 else if ((#2 (hd dias)) = mes) then let val flag = 1 + number_in_month(tl dias, mes) in flag end else number_in_month((tl dias),mes)
I hope you can also use it!
回答4:
The error message from REPL is confusing, but Jesper is right that you should use let-in-end expression, if you need assignment statement in functions. That will surely get you that error resolved.
来源:https://stackoverflow.com/questions/14409502/number-in-month-exercise