#!r6rs
( import ( rnrs base ( 6 ) )
( rnrs io simple ( 6 ) ) )
( define ( deriv exp var )
( define ( variable? x )
( symbol? x ) )
( define ( =number? exp num )
( and ( number? exp )
( = exp num ) ) )
( define ( same-variable? x1 x2 )
( and ( variable? x1 )
( variable? x2 )
( eq? x1 x2 ) ) )
( define ( make-sum a1 a2 )
( cond ( ( =number? a1 0 )
a2 )
( ( =number? a2 0 )
a1 )
( ( and ( number? a1 )
( number? a2 ) )
( + a1 a2 ) )
( else
( list '+ a1 a2 ) ) ) )
( define ( make-product m1 m2 )
( cond ( ( or ( =number? m1 0 )
( =number? m2 0 ) )
0 )
( ( =number? m1 1 )
m2 )
( ( =number? m2 1 )
m1 )
( ( and ( number? m1 )
( number? m2 ) )
( * m1 m2 ) )
( else
( list '* m1 m2 ) ) ) )
( define ( sum? x )
( and ( pair? x )
( eq? ( car x ) '+ ) ) )
( define ( addend s )
( cadr s ) )
( define ( augend s )
( caddr s ) )
( define ( product? x )
( and ( pair? x )
( eq? ( car x ) '* ) ) )
( define ( multiplier p )
( cadr p ) )
( define ( multiplicand p )
( caddr p ) )
( cond ( ( number? exp ) 0 )
( ( variable? exp )
( if ( same-variable? exp var ) 1
0 ) )
( ( sum? exp )
( make-sum ( deriv ( addend exp ) var )
( deriv ( augend exp ) var ) ) )
( ( product? exp )
( make-sum ( make-product ( multiplier exp )
( deriv ( multiplicand exp ) var ) )
( make-product ( deriv ( multiplier exp ) var )
( multiplicand exp ) ) ) )
( else
( error "unknown expression type: DERIV" exp ) ) ) )