package genprint

Genprint

A one function library and PPX extension to provide general value printing from anywhere within a program, as opposed to the ocaml toplevel behaviour of printing only evaluated expression results.

Used like so:

[%prs "text..." v]

or

[%pr some intro text followed by an expression (v,v) ]

where v is an arbitrary value. It forms a unit-valued expression.

The type of the value is retrieved from the containing source file's .cmt file which can be had via the compiler option [-bin-annot] or the recommended way of setting an environment variable permanently via one's .profile etc:

export OCAMLPARAM="_,-bin-annot=1"

This will ensure that all compilations generate annotation files, which are needed anyway for Merlin to function fully.

If using Dune or another build manager that places build artefacts other than alongside the source files then this environment variable needs setting:

export CMTPATH=<colon delimited list of directories to search>

With Dune a possible invocation might be:

CMTPATH=_build/default/test/.test.eobjs/byte dune exec ./test/test.exe

where a [test] directory contains some test source code.

The library is [genprint] and the PPX extension is [genprint.ppx], for both byte and optimising compilation.

See the test/dune file for Dune building with PPX. Otherwise, for example:

ocamlc -ppx '~/.opam/default/lib/genprint/ppx/ppx.exe --as-ppx' genprint.cma <src>

Limitations

This Genprint library cannot be used in the ocaml toplevel except in as much as loading objects already compiled with embedded printing statements and for which a .cmt file exists.

Genprint uses the compiler internals to do the actual printing and so will display <poly> for values than do not have an instantiated type (or for parts thereof). So avoid embedding this printer into a polymorphic context.

With the optimising compiler, the printing will fail (segmentation fault likely) where the assigned type in the .cmt does not correspond to the actual value given as argument to [%pr]. This may come about if the wrong .cmt file is retrieved due to searching in an incorrect directory that happens to have an identically named file but which is unrelated otherwise.

Lament For The -plugin Option

Having implemented Genprint as a compiler variant, then as this library, I became aware of the -plugin option and re-implemented to suit. It meant value types (to guide the printing) no longer came from .cmt files (therefore no CMTPATH setup), and no need of a PPX extension, just

open Genprint
   ...
   prs "some label..." x;
   ...

and which could be used within the toplevel too.

But then I discovered -plugin was scheduled to be removed from the compiler as of 4.09! What a pity. I hope another means of compiler extension will come in the future that doesn't involve creating a new compiler binary (which is anti-compositionality!)