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Dicken's avatar
Dicken
Post Prodigy
1 year ago
Solved

Transform Column / Type



Hello, 
Can some one explain this ;   

= Table.TransformColumns( Source, 
{ "Date", each Date.From(_)    }

which does what it say   "date"  now = a date ; 
but if I dispense with the full syntax 

= Table.TransformColumns( Source, 
{ "Date", Date.From } )

I now also transforms the date type ?  which is useful, just wondered why  ? 

Richard.

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  • MasonMA's avatar
    MasonMA
    1 year ago

    Hi Dicken 

     

    I think it's the behavior of Table.TransformColumns. If you see 

    Table.TransformColumns - Table Function | Power Query M

    this function decides how to apply the transformation function to each value or whether to update the column type metadata.

     

    In { "Date", each Date.From(_) }, Table.TransformColumns only sees a custom lambda, “(x) => Date.From(x)”. It doesn’t know the return type of this lambda, so it leaves the column type metadata unchanged.

    While in { "Date", Date.From }, Table.TransformColumns sees a native function that has a declared return type in the M library (type date). Since it knows that return type, it sets the column’s metadata to date as well.

     

    Ran a simple test below, 

    let
    Query1 = 
  let
    Source = #table(type table [Date = text], {{"2025-08-22"}}),
    Transform1 = Table.TransformColumns(Source, {"Date", each Date.From(_)}),
    Transform2 = Table.TransformColumns(Source, {"Date", Date.From})
  in
    { Transform1, Transform2 }
in
    Query1

3 Replies

  • MasonMA's avatar
    MasonMA
    Super User
    1 year ago

    Hi Dicken 

     

    I think it's the behavior of Table.TransformColumns. If you see 

    Table.TransformColumns - Table Function | Power Query M

    this function decides how to apply the transformation function to each value or whether to update the column type metadata.

     

    In { "Date", each Date.From(_) }, Table.TransformColumns only sees a custom lambda, “(x) => Date.From(x)”. It doesn’t know the return type of this lambda, so it leaves the column type metadata unchanged.

    While in { "Date", Date.From }, Table.TransformColumns sees a native function that has a declared return type in the M library (type date). Since it knows that return type, it sets the column’s metadata to date as well.

     

    Ran a simple test below, 

    let
    Query1 = 
  let
    Source = #table(type table [Date = text], {{"2025-08-22"}}),
    Transform1 = Table.TransformColumns(Source, {"Date", each Date.From(_)}),
    Transform2 = Table.TransformColumns(Source, {"Date", Date.From})
  in
    { Transform1, Transform2 }
in
    Query1