Julin S (Dec 07 2021 at 03:45): Julin S (Dec 07 2021 at 03:45):How can we obtain the composition of two functions for use with List.map? Do we always have to use compose or can we use function application with parenthesis?
I was trying to apply the List.map function to a list of tuples (2 nats each) like
Definition lst := [(1,10); (2,20); (3,30)].
Definition bar (n : nat) := n + 100.
I wanted to take the second element of each tuple and apply the bar function on it to produce a new list.
I tried
Compute List.map (bar snd) lst.
(*
The term "snd" has type "?A0 * ?B0 -> ?B0" while it is expected to have type
"nat".
*)
Type of snd is (?A * ?B) -> ?B which in the case of lst would become (nat * nat) -> nat.
And type of bar is nat -> nat.
It worked when I tried using compose from Coq.Program.Basics.
Compute List.map (compose bar snd) lst.
(*
= [110; 120; 130]
: list nat
*)
Is function composition not possible by simply writing them together? Do we always need compose?
Paolo Giarrusso (Dec 07 2021 at 06:08):Indeed, function composition is compose and not application; the type signature of compose describes exactly the job of composition.
Ali Caglayan (Dec 08 2021 at 10:35):compose exists for proof search. i.e. for typeclass search to be able to recognise a specific composition and find instances that way. You can also write fun x => bar (snd x)) for that function.
Last updated: Jan 29 2023 at 05:03 UTC