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Standard ML : trying to get the right signature of a function

Im trying to make a function with this signature :

'a->'b->('a * 'b ->'b)->'b

I have tried to do a function like this : 

fun f x y z = z(x,y);

but its signature is different , it is :

'a->'b->('a * 'b ->'c)->'c

How can i make it fit with the requested? Any tip maybe?

Solution

  • I think the least "artificial" would be something like:

    fun f x y g = g (x, g (x, y))

    which ensures that g's return-type matches the type of its second parameter by calling g twice, with the result of one call being the second argument to the other.

    But a simpler solution, if this isn't "cheating", is to just add an explicit type annotation:

    fun f x (y : 'b) g : 'b = g (x, y)