I am a beginner in SML. It's something easy but it took like hours and yet I cant solve it.
I have to take a list of string and invert every string as well as list. and do everything without defining multiple functions outside the main. e.g. ["stack","overflow","nice"] would give ["ecin","wolfrevo","kcats"]
fun oppositelist(l)=foldl (op::) nil (map ((fn(x)=>(implode o rev o explode)(x)), l));
and the error message is:
Error: operator and operand don't agree [tycon mismatch]
operator domain: 'Z -> 'Y
operand: (string -> string) * 'X
in expression:
map ((fn x => <exp> <exp>),l)
Don't solve everything, if the method is wrong. Hint it. Its a homework question.
Thank You.
The SML function map is curried. To map a function f over every element of the list l, you must write map f l. What you wrote is map (f,l), which means "map the function (f,l) over... something to be determined later." The compiler is complaining at you because (f,l) is not a function ('Z -> 'Y); it's a tuple consisting of a function and a list ((string -> string) * 'X).
I haven't looked any closer at your solution, but if you comment in response to this answer and ask me to, I probably will. :)
EDIT: Oh, and (fn(x)=>(implode o rev o explode)(x)) is just a long-winded way of writing (implode o rev o explode), for the same reason that writing (fn(x)=>f(x)) is a long-winded way of writing f.