Call:
rpart(formula = y ~ x, data = dat, method = "anova")
  n= 500 

          CP nsplit rel error    xerror        xstd
1 0.45512351      0 1.0000000 1.0029952 0.073143806
2 0.27010283      1 0.5448765 0.5925988 0.041461331
3 0.06436592      2 0.2747737 0.3004278 0.016810959
4 0.03867098      3 0.2104077 0.2508918 0.014572838
5 0.02320569      4 0.1717368 0.1870584 0.011369221
6 0.01645228      5 0.1485311 0.1704227 0.010007412
7 0.01000000      6 0.1320788 0.1566766 0.009218977

Node number 1: 500 observations,    complexity param=0.4551235
  mean=0.004169543, MSE=11.61044 
  left son=2 (111 obs) right son=3 (389 obs)
  Primary splits:
      x < -0.5571142  to the left,  improve=0.4551235, (0 missing)

Node number 2: 111 observations,    complexity param=0.06436592
  mean=-4.299136, MSE=5.241124 
  left son=4 (35 obs) right son=5 (76 obs)
  Primary splits:
      x < -0.8617235  to the left,  improve=0.642284, (0 missing)

Node number 3: 389 observations,    complexity param=0.2701028
  mean=1.232105, MSE=6.635892 
  left son=6 (325 obs) right son=7 (64 obs)
  Primary splits:
      x < 0.745491    to the left,  improve=0.6074335, (0 missing)

Node number 4: 35 observations
  mean=-7.002774, MSE=1.892769 

Node number 5: 76 observations
  mean=-3.05404, MSE=1.866575 

Node number 6: 325 observations,    complexity param=0.03867098
  mean=0.3411674, MSE=2.394114 
  left son=12 (140 obs) right son=13 (185 obs)
  Primary splits:
      x < 0.004008016 to the left,  improve=0.2885198, (0 missing)

Node number 7: 64 observations,    complexity param=0.02320569
  mean=5.756398, MSE=3.676083 
  left son=14 (40 obs) right son=15 (24 obs)
  Primary splits:
      x < 0.9058116   to the left,  improve=0.5725952, (0 missing)

Node number 12: 140 observations
  mean=-0.6142255, MSE=1.370827 

Node number 13: 185 observations,    complexity param=0.01645228
  mean=1.064167, MSE=1.955015 
  left son=26 (142 obs) right son=27 (43 obs)
  Primary splits:
      x < 0.5731463   to the left,  improve=0.2640723, (0 missing)

Node number 14: 40 observations
  mean=4.63259, MSE=1.443288 

Node number 15: 24 observations
  mean=7.629412, MSE=1.784321 

Node number 26: 142 observations
  mean=0.6687768, MSE=1.591589 

Node number 27: 43 observations
  mean=2.369876, MSE=0.9340242