Functional Design and Architecture MEAP V09 by copyright-2023-manning-publications.html

Author:copyright-2023-manning-publications.html
Language: eng
Format: epub

The forall keyword is what makes LogicControlMethod be GADT. The result type variable can’t escape the EvalHdl method, it’s localized there, and should be only accessed with the method itself. If we place this variable into the type itself, it will get a different meaning:

data LogicControlMethod result next = EvalHdl (Hdl result) (result -> next) type LogicControl result a = Free (LogicControlMethod result) a

No GADTs are now required, but look: the result type variable is one for the whole ADT, and it propagates upper into LogicControl. This means that whenever we call a LogicControl script, all the Hdl insertings are obligated to return the same type result. This will not compile:

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