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integrating pretty printing into the language and repl machinery

From: Justin Veilleux
Subject: integrating pretty printing into the language and repl machinery
Date: Mon, 20 Sep 2021 11:01:31 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Icedove/78.12.0

Hi guys. Recently, I wrote a lambda calculus parser and interpreter using guile's awesome compiler tower. It works relatively well, I'm able to enter expressions in the REPL and they are correctly executed.

However, because of lambda calculus' nature, the interactive experience is far from fun. Since everything is a function, when I try and test my definition of addition of church encoded numbers, I'm met with an unfriendly `#<procedure>'.

I wrote a `render-function' procedure which, given an #<procedure> from the lambda calculus world, will return a friendly representation such as `λa.λb.(b (b a))'

I thought, "there must be a way to convince the REPL to use this function instead of the unhelpful `write'".

While looking at the spec.scm file in language/ecmascript, I saw the ";; a pretty printer would be interesting" comment and was convinced that the #:printer field was my solution, but after investigating further, realized that it wasn't. In fact, if I understand correctly, there is currently no way of telling the REPL how I want the result of an expression printed.

I have two questions.

1. What was the motivation for the #:printer slot in (system base language), if it isn't supposed to be used to print the result of evaluation (according to (repl common)) ?

2. If I were to do a bit of hacking, maybe add a #:pretty-printer slot and integrate it with the REPL, is that something that could get merged?

Thank you.

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