Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
sig type attribute = string Asttypes.loc * Parsetree.payload and extension = string Asttypes.loc * Parsetree.payload and attributes = Parsetree.attribute list and payload = PStr of Parsetree.structure | PTyp of Parsetree.core_type | PPat of Parsetree.pattern * Parsetree.expression option and core_type = { ptyp_desc : Parsetree.core_type_desc; ptyp_loc : Location.t; ptyp_attributes : Parsetree.attributes; } and core_type_desc = Ptyp_any | Ptyp_var of string | Ptyp_arrow of Asttypes.label * Parsetree.core_type * Parsetree.core_type | Ptyp_tuple of Parsetree.core_type list | Ptyp_constr of Longident.t Asttypes.loc * Parsetree.core_type list | Ptyp_object of (string * Parsetree.attributes * Parsetree.core_type) list * Asttypes.closed_flag | Ptyp_class of Longident.t Asttypes.loc * Parsetree.core_type list | Ptyp_alias of Parsetree.core_type * string | Ptyp_variant of Parsetree.row_field list * Asttypes.closed_flag * Asttypes.label list option | Ptyp_poly of string list * Parsetree.core_type | Ptyp_package of Parsetree.package_type | Ptyp_extension of Parsetree.extension and package_type = Longident.t Asttypes.loc * (Longident.t Asttypes.loc * Parsetree.core_type) list and row_field = Rtag of Asttypes.label * Parsetree.attributes * bool * Parsetree.core_type list | Rinherit of Parsetree.core_type and pattern = { ppat_desc : Parsetree.pattern_desc; ppat_loc : Location.t; ppat_attributes : Parsetree.attributes; } and pattern_desc = Ppat_any | Ppat_var of string Asttypes.loc | Ppat_alias of Parsetree.pattern * string Asttypes.loc | Ppat_constant of Asttypes.constant | Ppat_interval of Asttypes.constant * Asttypes.constant | Ppat_tuple of Parsetree.pattern list | Ppat_construct of Longident.t Asttypes.loc * Parsetree.pattern option | Ppat_variant of Asttypes.label * Parsetree.pattern option | Ppat_record of (Longident.t Asttypes.loc * Parsetree.pattern) list * Asttypes.closed_flag | Ppat_array of Parsetree.pattern list | Ppat_or of Parsetree.pattern * Parsetree.pattern | Ppat_constraint of Parsetree.pattern * Parsetree.core_type | Ppat_type of Longident.t Asttypes.loc | Ppat_lazy of Parsetree.pattern | Ppat_unpack of string Asttypes.loc | Ppat_exception of Parsetree.pattern | Ppat_extension of Parsetree.extension and expression = { pexp_desc : Parsetree.expression_desc; pexp_loc : Location.t; pexp_attributes : Parsetree.attributes; } and expression_desc = Pexp_ident of Longident.t Asttypes.loc | Pexp_constant of Asttypes.constant | Pexp_let of Asttypes.rec_flag * Parsetree.value_binding list * Parsetree.expression | Pexp_function of Parsetree.case list | Pexp_fun of Asttypes.label * Parsetree.expression option * Parsetree.pattern * Parsetree.expression | Pexp_apply of Parsetree.expression * (Asttypes.label * Parsetree.expression) list | Pexp_match of Parsetree.expression * Parsetree.case list | Pexp_try of Parsetree.expression * Parsetree.case list | Pexp_tuple of Parsetree.expression list | Pexp_construct of Longident.t Asttypes.loc * Parsetree.expression option | Pexp_variant of Asttypes.label * Parsetree.expression option | Pexp_record of (Longident.t Asttypes.loc * Parsetree.expression) list * Parsetree.expression option | Pexp_field of Parsetree.expression * Longident.t Asttypes.loc | Pexp_setfield of Parsetree.expression * Longident.t Asttypes.loc * Parsetree.expression | Pexp_array of Parsetree.expression list | Pexp_ifthenelse of Parsetree.expression * Parsetree.expression * Parsetree.expression option | Pexp_sequence of Parsetree.expression * Parsetree.expression | Pexp_while of Parsetree.expression * Parsetree.expression | Pexp_for of Parsetree.pattern * Parsetree.expression * Parsetree.expression * Asttypes.direction_flag * Parsetree.expression | Pexp_constraint of Parsetree.expression * Parsetree.core_type | Pexp_coerce of Parsetree.expression * Parsetree.core_type option * Parsetree.core_type | Pexp_send of Parsetree.expression * string | Pexp_new of Longident.t Asttypes.loc | Pexp_setinstvar of string Asttypes.loc * Parsetree.expression | Pexp_override of (string Asttypes.loc * Parsetree.expression) list | Pexp_letmodule of string Asttypes.loc * Parsetree.module_expr * Parsetree.expression | Pexp_assert of Parsetree.expression | Pexp_lazy of Parsetree.expression | Pexp_poly of Parsetree.expression * Parsetree.core_type option | Pexp_object of Parsetree.class_structure | Pexp_newtype of string * Parsetree.expression | Pexp_pack of Parsetree.module_expr | Pexp_open of Asttypes.override_flag * Longident.t Asttypes.loc * Parsetree.expression | Pexp_extension of Parsetree.extension and case = { pc_lhs : Parsetree.pattern; pc_guard : Parsetree.expression option; pc_rhs : Parsetree.expression; } and value_description = { pval_name : string Asttypes.loc; pval_type : Parsetree.core_type; pval_prim : string list; pval_attributes : Parsetree.attributes; pval_loc : Location.t; } and type_declaration = { ptype_name : string Asttypes.loc; ptype_params : (Parsetree.core_type * Asttypes.variance) list; ptype_cstrs : (Parsetree.core_type * Parsetree.core_type * Location.t) list; ptype_kind : Parsetree.type_kind; ptype_private : Asttypes.private_flag; ptype_manifest : Parsetree.core_type option; ptype_attributes : Parsetree.attributes; ptype_loc : Location.t; } and type_kind = Ptype_abstract | Ptype_variant of Parsetree.constructor_declaration list | Ptype_record of Parsetree.label_declaration list | Ptype_open and label_declaration = { pld_name : string Asttypes.loc; pld_mutable : Asttypes.mutable_flag; pld_type : Parsetree.core_type; pld_loc : Location.t; pld_attributes : Parsetree.attributes; } and constructor_declaration = { pcd_name : string Asttypes.loc; pcd_args : Parsetree.core_type list; pcd_res : Parsetree.core_type option; pcd_loc : Location.t; pcd_attributes : Parsetree.attributes; } and type_extension = { ptyext_path : Longident.t Asttypes.loc; ptyext_params : (Parsetree.core_type * Asttypes.variance) list; ptyext_constructors : Parsetree.extension_constructor list; ptyext_private : Asttypes.private_flag; ptyext_attributes : Parsetree.attributes; } and extension_constructor = { pext_name : string Asttypes.loc; pext_kind : Parsetree.extension_constructor_kind; pext_loc : Location.t; pext_attributes : Parsetree.attributes; } and extension_constructor_kind = Pext_decl of Parsetree.core_type list * Parsetree.core_type option | Pext_rebind of Longident.t Asttypes.loc and class_type = { pcty_desc : Parsetree.class_type_desc; pcty_loc : Location.t; pcty_attributes : Parsetree.attributes; } and class_type_desc = Pcty_constr of Longident.t Asttypes.loc * Parsetree.core_type list | Pcty_signature of Parsetree.class_signature | Pcty_arrow of Asttypes.label * Parsetree.core_type * Parsetree.class_type | Pcty_extension of Parsetree.extension and class_signature = { pcsig_self : Parsetree.core_type; pcsig_fields : Parsetree.class_type_field list; } and class_type_field = { pctf_desc : Parsetree.class_type_field_desc; pctf_loc : Location.t; pctf_attributes : Parsetree.attributes; } and class_type_field_desc = Pctf_inherit of Parsetree.class_type | Pctf_val of (string * Asttypes.mutable_flag * Asttypes.virtual_flag * Parsetree.core_type) | Pctf_method of (string * Asttypes.private_flag * Asttypes.virtual_flag * Parsetree.core_type) | Pctf_constraint of (Parsetree.core_type * Parsetree.core_type) | Pctf_attribute of Parsetree.attribute | Pctf_extension of Parsetree.extension and 'a class_infos = { pci_virt : Asttypes.virtual_flag; pci_params : (Parsetree.core_type * Asttypes.variance) list; pci_name : string Asttypes.loc; pci_expr : 'a; pci_loc : Location.t; pci_attributes : Parsetree.attributes; } and class_description = Parsetree.class_type Parsetree.class_infos and class_type_declaration = Parsetree.class_type Parsetree.class_infos and class_expr = { pcl_desc : Parsetree.class_expr_desc; pcl_loc : Location.t; pcl_attributes : Parsetree.attributes; } and class_expr_desc = Pcl_constr of Longident.t Asttypes.loc * Parsetree.core_type list | Pcl_structure of Parsetree.class_structure | Pcl_fun of Asttypes.label * Parsetree.expression option * Parsetree.pattern * Parsetree.class_expr | Pcl_apply of Parsetree.class_expr * (Asttypes.label * Parsetree.expression) list | Pcl_let of Asttypes.rec_flag * Parsetree.value_binding list * Parsetree.class_expr | Pcl_constraint of Parsetree.class_expr * Parsetree.class_type | Pcl_extension of Parsetree.extension and class_structure = { pcstr_self : Parsetree.pattern; pcstr_fields : Parsetree.class_field list; } and class_field = { pcf_desc : Parsetree.class_field_desc; pcf_loc : Location.t; pcf_attributes : Parsetree.attributes; } and class_field_desc = Pcf_inherit of Asttypes.override_flag * Parsetree.class_expr * string option | Pcf_val of (string Asttypes.loc * Asttypes.mutable_flag * Parsetree.class_field_kind) | Pcf_method of (string Asttypes.loc * Asttypes.private_flag * Parsetree.class_field_kind) | Pcf_constraint of (Parsetree.core_type * Parsetree.core_type) | Pcf_initializer of Parsetree.expression | Pcf_attribute of Parsetree.attribute | Pcf_extension of Parsetree.extension and class_field_kind = Cfk_virtual of Parsetree.core_type | Cfk_concrete of Asttypes.override_flag * Parsetree.expression and class_declaration = Parsetree.class_expr Parsetree.class_infos and module_type = { pmty_desc : Parsetree.module_type_desc; pmty_loc : Location.t; pmty_attributes : Parsetree.attributes; } and module_type_desc = Pmty_ident of Longident.t Asttypes.loc | Pmty_signature of Parsetree.signature | Pmty_functor of string Asttypes.loc * Parsetree.module_type option * Parsetree.module_type | Pmty_with of Parsetree.module_type * Parsetree.with_constraint list | Pmty_typeof of Parsetree.module_expr | Pmty_extension of Parsetree.extension | Pmty_alias of Longident.t Asttypes.loc and signature = Parsetree.signature_item list and signature_item = { psig_desc : Parsetree.signature_item_desc; psig_loc : Location.t; } and signature_item_desc = Psig_value of Parsetree.value_description | Psig_type of Parsetree.type_declaration list | Psig_typext of Parsetree.type_extension | Psig_exception of Parsetree.extension_constructor | Psig_module of Parsetree.module_declaration | Psig_recmodule of Parsetree.module_declaration list | Psig_modtype of Parsetree.module_type_declaration | Psig_open of Parsetree.open_description | Psig_include of Parsetree.include_description | Psig_class of Parsetree.class_description list | Psig_class_type of Parsetree.class_type_declaration list | Psig_attribute of Parsetree.attribute | Psig_extension of Parsetree.extension * Parsetree.attributes and module_declaration = { pmd_name : string Asttypes.loc; pmd_type : Parsetree.module_type; pmd_attributes : Parsetree.attributes; pmd_loc : Location.t; } and module_type_declaration = { pmtd_name : string Asttypes.loc; pmtd_type : Parsetree.module_type option; pmtd_attributes : Parsetree.attributes; pmtd_loc : Location.t; } and open_description = { popen_lid : Longident.t Asttypes.loc; popen_override : Asttypes.override_flag; popen_loc : Location.t; popen_attributes : Parsetree.attributes; } and 'a include_infos = { pincl_mod : 'a; pincl_loc : Location.t; pincl_attributes : Parsetree.attributes; } and include_description = Parsetree.module_type Parsetree.include_infos and include_declaration = Parsetree.module_expr Parsetree.include_infos and with_constraint = Pwith_type of Longident.t Asttypes.loc * Parsetree.type_declaration | Pwith_module of Longident.t Asttypes.loc * Longident.t Asttypes.loc | Pwith_typesubst of Parsetree.type_declaration | Pwith_modsubst of string Asttypes.loc * Longident.t Asttypes.loc and module_expr = { pmod_desc : Parsetree.module_expr_desc; pmod_loc : Location.t; pmod_attributes : Parsetree.attributes; } and module_expr_desc = Pmod_ident of Longident.t Asttypes.loc | Pmod_structure of Parsetree.structure | Pmod_functor of string Asttypes.loc * Parsetree.module_type option * Parsetree.module_expr | Pmod_apply of Parsetree.module_expr * Parsetree.module_expr | Pmod_constraint of Parsetree.module_expr * Parsetree.module_type | Pmod_unpack of Parsetree.expression | Pmod_extension of Parsetree.extension and structure = Parsetree.structure_item list and structure_item = { pstr_desc : Parsetree.structure_item_desc; pstr_loc : Location.t; } and structure_item_desc = Pstr_eval of Parsetree.expression * Parsetree.attributes | Pstr_value of Asttypes.rec_flag * Parsetree.value_binding list | Pstr_primitive of Parsetree.value_description | Pstr_type of Parsetree.type_declaration list | Pstr_typext of Parsetree.type_extension | Pstr_exception of Parsetree.extension_constructor | Pstr_module of Parsetree.module_binding | Pstr_recmodule of Parsetree.module_binding list | Pstr_modtype of Parsetree.module_type_declaration | Pstr_open of Parsetree.open_description | Pstr_class of Parsetree.class_declaration list | Pstr_class_type of Parsetree.class_type_declaration list | Pstr_include of Parsetree.include_declaration | Pstr_attribute of Parsetree.attribute | Pstr_extension of Parsetree.extension * Parsetree.attributes and value_binding = { pvb_pat : Parsetree.pattern; pvb_expr : Parsetree.expression; pvb_attributes : Parsetree.attributes; pvb_loc : Location.t; } and module_binding = { pmb_name : string Asttypes.loc; pmb_expr : Parsetree.module_expr; pmb_attributes : Parsetree.attributes; pmb_loc : Location.t; } type toplevel_phrase = Ptop_def of Parsetree.structure | Ptop_dir of string * Parsetree.directive_argument and directive_argument = Pdir_none | Pdir_string of string | Pdir_int of int | Pdir_ident of Longident.t | Pdir_bool of bool end