Historical Linguistics and Language Families 7, Exercise 3: the will cline

willan (Old English "to want, to wish")   content word
will (auxiliary for intention, then future) grammatical word
'll (reduced form in speech)               clitic

Mechanisms at work: semantic bleaching (the meaning "to wish" fades and only
future or intention remains) and phonetic erosion (will reduces to 'll). The
mapping of 'll to the clitic stage is my reading of the cline; the source
states the cline and the will example separately.

For unidirectionality: Heine calls grammaticalisation "a unidirectional
process", and the change generally runs from less to more grammatical.
Against: critics point to counterexamples such as degrammaticalisation in
North Saami, and Richard Janda cites over 70 works critical of the
hypothesis; Lyle Campbell says advocates often minimise or redefine these
cases.