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.

quoting the wiki link provided upthread:

> ML (Meta Language) is a general-purpose functional programming language. It has roots in Lisp, and has been characterized as "Lisp with types". It is known for its use of the polymorphic Hindley–Milner type system, which automatically assigns the types of most expressions without requiring explicit type annotations, and ensures type safety – there is a formal proof that a well-typed ML program does not cause runtime type errors.

The original ML was implemented in Lisp (Stanford Lisp) as part of the Edinburgh LCF (Logic for Computable Functions) theorem prover. Different syntax, types - but other stuff it had were easily shared in the implementation with the underlying Lisp: functions, higher-order functions, garbage collection, lists (with cons and append operators), lambda expressions, ...

Here is a version of LCF with that first ML from 1977: https://github.com/theoremprover-museum/LCF77