The original ML language (the one produced as part of the LCF theorem prover), did have a `where` clause that works like the one in Haskell. See here for example: https://github.com/theoremprover-museum/LCF77/blob/master/sr...

A brief skim over the revised definition of Standard ML indicates that it was abandoned at some point. I've no idea if Caml or OCaml kept it.

That said, I'm reasonably sure that the syntactic construct (insofar is it is used in programming languages) originated with the ISWIM introduced in this paper, which was rather influential in its day, perhaps especially in the functional programming community.