PowerShell DSLs: Using hashtables and scriptblocks together

Previously I presented the use of hashtables and scriptblocks as DSL input formats. You can also convert between them, with processing in between. I tried that out, and I think the result is interesting enough to post. Again, here is the input:

class MyClass {

    field ([string]) S

}

The previous code would use the 'field' function to manipulate the class object as appropriate:

    function field

    {

        param (

            [Type] $type,

            [String] $name

        )

        

        # add the field declaration

        $field = New-Object System.CodeDom.CodeMemberField($type, $name)

        $field.Attributes = [System.CodeDom.MemberAttributes]::Public

        $class.Members.Add( $field )

        

        # add the ctor parameter

        $ctorParameter = New-Object System.CodeDom.CodeParameterDeclarationExpression($type, $name)

        $constructor.Parameters.Add($ctorParameter)

 

        # add the ctor initializer

        $fieldReference = New-Object System.CodeDom.CodeFieldReferenceExpression(

            (New-Object System.CodeDom.CodeThisReferenceExpression),

            $name

        )

        

        $ctorInitializer = New-Object System.CodeDom.CodeAssignStatement(

            $fieldReference,

            (New-Object System.CodeDom.CodeVariableReferenceExpression $name)

        )

        

        $constructor.Statements.Add( $ctorInitializer )

    }

    

    & $memberScriptBlock | Out-Null

 

But another idea is to have the 'field' function produce a collection of CodeDom objects that can be assembled later.

    # return a hash of the CodeDom objects related to

    # this field

    function field

    {

        param (

            [Type] $type,

            [String] $name

        )

        

        @{

            fieldDeclaration = $(

                $field = New-Object System.CodeDom.CodeMemberField($type, $name)

                $field.Attributes = [System.CodeDom.MemberAttributes]::Public

                $field

                )

            parameter = New-Object System.CodeDom.CodeParameterDeclarationExpression($type, $name)

            fieldReference = New-Object System.CodeDom.CodeFieldReferenceExpression(

                (New-Object System.CodeDom.CodeThisReferenceExpression),

                $name

            )

            parameterReference = New-Object System.CodeDom.CodeVariableReferenceExpression $name

        }                

    }

    

    & $memberScriptBlock | foreach {

        $class.Members.Add( $_.fieldDeclaration )

        

        $constructor.Parameters.Add( $_.parameter )

 

        $constructor.Statements.Add(

            $(New-Object System.CodeDom.CodeAssignStatement( $_.fieldReference, $_.parameterReference ))

        )

    } | Out-Null

I suspect that the latter model is a little better because it separates concerns. Consider if I were to add other statements to my language, such as 'property'. The 'foreach' at the end could probably be written in a way that works for both fields and properties. However, the hashtable is slightly concerning, because it's not typed – if I get the key names wrong somewhere, I'm screwed.

I find that I'm spending quite a lot of time on this, but it's important that I find a way to create DSLs quickly. I figured that right now I'm just learning the techniques, and then if I master them, then I can do it more quickly when the time comes.