Filtering OCaml list to one variant -

so have list of stmt (algebraic type) contain number of vardecl within list. i'd reduce list stmt list vardecl list. when use list.filter can eliminate other types i'm still left stmt list.

i found able filtering type change folding, can't figure out how generalize (i need pattern many places in project).

let decls = list.fold_left     (fun lst st -> match st         | vardecl(vd) -> vd :: lst         | _ -> lst     ) [] stmts in 

is there better way perform filter , cast variant of list type?

assuming have type like

type stmt = vardecl of int | foo of int | bar | fie of string 

and stmt list, batteries lets do

let vardecl_ints l =     list.filter_map (function vardecl -> | _ -> none) l  let foo_ints l =     list.filter_map (function foo -> | _ -> none) l 

which think concise you're going get. don't think can make general "list-getters" adt's, because e.g.

let bars l =     list.filter_map (function bar -> bar | _ -> none) l 

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https://github.com/ocaml-batteries-team/batteries-included/blob/master/src/batlist.mlv#l456 has batteries implementation of filter_map, if don't want dependency. functional version [] instead of acc quite similar, doing (x::dst) , |>list.rev @ end.