I have a little bit of a problem in understanding the function map with typed/racket. My problem starts with an application of map which in normal racket is very straight forward: > (map car '(("Eins" "One" "Un") ("Zwei" "Dos" "Two" "Deux") ("Drei" "Tres" "Trois"))) '("Eins" "Zwei" "Drei") If map is applied this way in typed/racket it yields an error: Type Checker: Polymorphic function `map' could not be applied to arguments: Domains: (-> a b ... b c) (Listof a) (Listof b) ... b (-> a c) (Pairof a (Listof a)) Arguments: (All (a b) (case-> (-> (Pairof a b) a) (-> (Listof a) a))) (List (List String String String) (List String String String String) (List String String String)) There is an easy way to get around this by using for/list with some type annotations. > (let ([ml '(("Eins" "One" "Un") ("Zwei" "Dos" "Two" "Deux") ("Drei" "Tres" "Trois"))]) (for/list : (Listof String) ([l : (Listof String) ml]) (car l))) - : (Listof String) '("Eins" "Zwei" "Drei") But this leaves me a little bit irritated. The function map has the following type in typed/racket: > map - : (All (c a b ...) (case-> (-> (-> a c) (Pairof a (Listof a)) (Pairof c (Listof c))) (-> (-> a b ... b c) (Listof a) (Listof b) ... b (Listof c)))) #<procedure:map> So in my code here I believed the type system would pick the first case where map is a function that takes two arguments a function with the domain of a and the range of c and as second argument a not emtpy list of type a. I thought the type placeholder a could in this case be (Listof String) so (Pairof a (Listof a)) translates to (Pairof String (Listof String)) which is the same as a nonempty (Listof String). But apparantly the type system has a different view and is unable to apply map to the supplied arguments. So I do need some help to get a better understanding of the type system. Thanks to everyone who reads this.

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