M.V. Hutz
528956b033
Add a new interpreter option (-i debruijn) that uses De Bruijn indices for variable representation, eliminating the need for variable renaming during substitution. - Add -i flag to select interpreter (lambda or debruijn) - Create debruijn package with Expression types (Variable with index, Abstraction without parameter, Application) - Implement shift and substitute operations for De Bruijn indices - Add conversion functions between lambda and De Bruijn representations - Update CLI to support switching between interpreters - Add De Bruijn tests to verify all samples pass Closes #26
33 lines
866 B
Go
33 lines
866 B
Go
package debruijn
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// Shift increments all free variable indices in an expression by the given amount.
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// A variable is free if its index is >= the cutoff (depth of nested abstractions).
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// This is necessary when substituting an expression into a different binding context.
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func Shift(expr Expression, amount int, cutoff int) Expression {
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switch e := expr.(type) {
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case *Variable:
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if e.index >= cutoff {
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return NewVariable(e.index+amount, e.label)
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}
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return e
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case *Abstraction:
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newBody := Shift(e.body, amount, cutoff+1)
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if newBody == e.body {
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return e
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}
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return NewAbstraction(newBody)
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case *Application:
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newAbs := Shift(e.abstraction, amount, cutoff)
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newArg := Shift(e.argument, amount, cutoff)
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if newAbs == e.abstraction && newArg == e.argument {
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return e
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}
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return NewApplication(newAbs, newArg)
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default:
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return expr
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}
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}
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