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;;; This handles the default rule.
(define (maybe-default-ambiguous-tyvar type def module)
(let ((classes (ntyvar-context type)))
(and (not (null? classes)) ; this happens only during cleanup after an error
(let ((non-standard? '#f)
(numeric? '#f))
(dolist (class classes)
(cond ((eq? (class-kind class) 'numeric)
(setf numeric? '#t))
((not (eq? (class-kind class) 'standard))
(setf non-standard? '#t))))
(cond ((or non-standard? (not numeric?))
(remember-context def
(phase-error 'Non-defaultable-ambiguous-context
"An ambiguous context, ~A, cannot be defaulted.~%Ambiguity in call to ~A~%"
classes def))
'#f)
(else
(find-default-type type classes classes
(tuple-2-2 (assq module *default-decls*)))))))))
(define (find-default-type tyvar classes all-classes defaults)
(cond ((null? defaults)
(phase-error 'no-default-applies
"Ambiguous context: ~A~%No default applies.~%"
all-classes)
'#f)
((null? classes)
(instantiate-tyvar tyvar (car defaults))
'#t)
((type-in-class? (car defaults) (car classes))
(find-default-type tyvar (cdr classes) all-classes defaults))
(else
(find-default-type tyvar all-classes all-classes (cdr defaults)))))
(define (type-in-class? ntype class)
(let* ((ntype (expand-ntype-synonym ntype))
(alg (ntycon-tycon ntype))
(inst (lookup-instance alg class)))
(if (eq? inst '#f)
'#f
(let ((res '#t))
(do-contexts (c (instance-context inst)) (ty (ntycon-args ntype))
(when (not (type-in-class? ty c))
(setf res '#f)))
res))))
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