[Python] polars remember last value

I like to create indicators for stocks. For ewm_mean, diffences, cum_sum, when/than/otherwise works fine. But here is nut i can't crack. First step is to find the direction of the values.

import polars as pl

df = pl.DataFrame(
{
"index": [0,1,2,3,4,5,6,7,8,9,10,11],
"hl": [float("nan"),float("nan"),-1,float("nan"),1,float("nan"),float("nan"),float("nan"),-1,float("nan"),-1,float("nan")],
"thisresultwouldbefine": [float("nan"),float("nan"),-1,-1,1,1,1,1,-1,-1,-1,-1],
}
)
print(df)

df = df.with_columns(pl.col('hl').alias('result'))
print(df)

index hl thisresultwouldbefine
0
1
2 -1.0 -1.0
3 -1.0
4 1.0 1.0
5 1.0
6 1.0
7 1.0
8 -1.0 -1.0
9 -1.0
10 -1.0 -1.0
11 -1.0

index not needed (only for discussion). hl are the giving values. thisresultwouldbefine should be the output. At index 2 hl is -1. This should be memorized for the next value until hl changed. At index 3 hl is NaN and result should be -1 furthermore. At index 4 hl is 1 and the result should be 1.

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