I am trying to devise a pretty-printer for an Env.t type. An Env.t is simply a map from identifiers to values, i.e. a Value.t IdMap.t. let pp fmt env = let open Format in fprintf fmt "[ @[<v 2>"; env |> IdMap.iter (fun id val_ -> fprintf fmt "@[<hov 2>%s ->@ %a@]@." id Value.pp …

[image] otini: @. You are flushing the pretty printer. This closes all open boxes and starts over. You wanted to write @,. [image] otini: would using Fmt make my life easier? No. Except for a few nice additions, Fmt is essentially Format with the right names for concise definitions.

Thank you. Replacing with @, gives me [ result -> 42. succ -> <function> x -> 42. ] I am still unsure why.

I suspect you have an unclosed box in Value.pp.

I can’t find it. Value.pp is defined as: let pp fmt = let open Format in function | Nil -> pp_print_string fmt "nil" | Boolean b -> pp_print_bool fmt b | Number n -> pp_print_float fmt n | Function _ -> pp_print_string fmt "<function>" I’ll investigate more.

I think your boxing layout is wrong you need to write "@[<v2>[ " … "]@]". Currently you start you box after "[ " so it’s going to set the “box tab” at that column and will indent furthermore from there when you get on new lines.

[image] otini: What am I doing wrong I found the old tutorial https://caml.inria.fr/resources/doc/guides/format.en.html quite useful in understanding the semantics of Format. Cheers, Nicolas

Note that it is on the new website aswell.

fmt might help if you shift your focus to learning its ‘design language’. I wrote a little bit about that here: How to print anything in OCaml - DEV Community In your case suppose we want to format a type int Map.Make(String).t, i.e. a map from string to int. We can do: module Env = Map.Make(Strin…

Now @yawaramin is provoking me into offering a less wasteful answer – that is one that doesn’t go through Seq. module Env = Map.Make(String) let pp_binding = Fmt.(hbox @@ pair ~sep:comma string int) let pp_env = Fmt.(braces @@ vbox @@ iter_bindings ~sep:semi Env.iter pp_binding)

We eagerly await benchmarks exposing how wasteful the seq solution was.

Neither of your suggested fixes worked on their own, but I finally figured out that it was entirely my fault. I used \n in Format.printf before that. Yes, I know that the documentation says not to, but I thought that at the top level out of any box it was fine. Now thinking twice I can see why it is…

Will I ever understand Format? or How to print a list vertically with indentation

Learning

gaga August 27, 2022, 12:19pm 16

There is also a nice document called “Format Unraveled”, if you want to learn more about Format. It can be found here

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Will I ever understand Format? or How to print a list vertically with indentation

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