M1 contains metadata

The M1 type attaches metadata to the type it wraps. We see a lot of M1s in Generics representations, because every type, type constructor, and type parameter is wrapped in one.

The definition of M1 is:

newtype M1 kind meta f p = M1 (f p)

Depending on what is being wrapped we’ll encounter one of three pairs of kind and meta parameters.

wrapped type	kind	meta
a type	D	'MetaData typename module package isnewtype
a constructor	C	'MetaCons constructorname fixity isrecord
a parameter	S	'MetaSel fieldname unpacked strict strict2

As you can see there’s a lot of information attached here. To learn more about what it all means open the details pages for one of the metadata types.

Beware that M1 sometimes goes by the names D1, C1, and S1. These are type synonyms for M1 with the kind parameter pre-applied:

type D1 = M1 D
type C1 = M1 C
type S1 = M1 S