Because programming language talk is more interesting if you see some code, here is the very first example program from the list of programs in the tutorial [1]:

    fmod A-GRAPH is
       sorts Edge Node .
       ops n1 n2 n3 n4 n5 : -> Node .
       ops a b c d e f : -> Edge .
       ops source target : Edge -> Node .
    
       eq source(a) = n1 .  eq target(a) = n2 .
       eq source(b) = n1 .  eq target(b) = n3 .
       eq source(c) = n3 .  eq target(c) = n4 .
       eq source(d) = n4 .  eq target(d) = n2 .
       eq source(e) = n2 .  eq target(e) = n5 .
       eq source(f) = n2 .  eq target(f) = n1 .
    endfm

Edit: The title of the above code is "A concrete graph".

This is very much not my cup of tea, but it seems to have been around for a while so I guess it's filling its purpose. Would be cool to hear from HN users who drive this daily.

[1]: http://maude.cs.uiuc.edu/maude1/tutorial/CoreMaudeExamples/a...

I toyed with Maude many years ago (e.g. to play with the BitBitJump language https://github.com/Warbo/bitbitjump-maude/blob/master/bbj.ma... ).

The Igor system for inductive functional programming (generating pure-functional programs which satisfy examples) was written in Maude, although version 2 was ported to Haskell https://www.researchgate.net/publication/221247917_Porting_I...

Need to learn it some time. I hope at some point it is ported to racket or some other lisp for proper maintenance.

Maude, eh?